Pool Algae Types: Green, Mustard & Black (Kill Guide)

Not all pool algae is the same green stuff. In Pinellas County we deal with three real types — green, mustard (yellow), and black — plus a bacterial imposter that looks like algae but isn't. Each has a different cause and a different kill sequence. Treat black algae like a green bloom and you'll waste a week and a lot of chlorine. Here's how to identify what's in your pool and exactly how to get rid of it.

Green algae — the fast, forgiving one

Green algae is free-floating, which is why it turns the whole water column green so fast — clear on Monday, pea-soup by Thursday. It clings loosely to walls and steps but wipes off with almost no effort. The good news: it's the easiest type to kill.

Cause: almost always low or zero free chlorine, made worse by high phosphates and poor circulation. A stalled pump, an empty chlorinator, or a storm that dilutes your chemistry is the usual trigger.

Kill sequence:

1. Test and balance pH into the 7.2–7.6 range first — shock is far less effective at high pH.
2. Triple shock (three times the normal dose) with cal-hypo or liquid chlorine at dusk.
3. Brush every wall, step, and floor to break algae loose into the water.
4. Run the pump a full 24 hours so the filter can pull the dead algae out.
5. Clean the filter, re-test, and hold free chlorine at 3+ ppm until the water is clear.

Most light-to-medium green pools clear in 24–72 hours this way. If you can still see the bottom, this is a DIY job — we lay out the full timing in our 48-hour green pool recovery guide.

Mustard (yellow) algae — the one that keeps coming back

Mustard algae looks like a dusting of sand or pollen, usually on shaded walls and in corners where circulation is weak. The tell: you brush it off and it reappears in the same spot a day or two later. That's because it's chlorine-resistant and hides in more than just the plaster.

Cause:mustard algae survives normal chlorine levels, so it thrives even in pools that look otherwise healthy. It contaminates equipment, ladders, floats, nets, and swimwear — anything that goes in the water becomes a re-infection source.

Kill sequence:

• Elevate the shock well above a green-algae dose and pair it with a mustard-specific (yellow-treat) algaecide — standard algaecide won't do it.
• Brush every surface, including behind lights and ladders, right after dosing.
• Decontaminate everything that touches the pool: run pool toys, floats, nets, and the pole through the shocked water, and wash swimsuits. Replace cheap accessories that won't come clean.
• Hold elevated chlorine for 48–72 hours — mustard needs sustained pressure, not a single hit.

Skip the equipment decontamination and mustard comes back every time, no matter how much you shock. If it survives two full rounds, bring in our algae treatment service — we treat the pool and the equipment as one system.

Black algae — the hardest to kill

Black algae shows up as small, raised dark-green or black heads on plaster or in grout lines, usually on a rough or aging surface. Unlike green and mustard, it isn't floating — it roots into the surface and grows a waxy protective layer that shrugs off chlorine. You cannot just shock it away.

Kill sequence:

1. Break the heads first. Scrub aggressively with a stiff wire or black-algae brush on plaster (use a nylon brush on vinyl or fiberglass) to tear off the protective cap and expose the roots.
2. Spot-treat the roots. Rub a broken chlorine tablet directly on each spot, or apply granular chlorine or a copper-based algaecide to the exact locations.
3. Shock and circulate the whole pool, brushing daily.
4. Repeat over 1–2 weeks. Black algae almost always needs multiple rounds — miss one root and it regrows.

Recurring black algae usually signals a porous or aging surface that gives the roots somewhere to hide. This is the type we most often recommend handing to a pro — it takes aggressive brushing, repeat visits, and an honest read on whether the surface itself is the real problem.

Pink slime & white water mold — not actually algae

If you find pink or reddish slime on fittings, or grayish-white tissue-like clumps near the returns, that's not algae at all — it's bacteria and a fungal biofilm that feels slimy rather than dusty and favors shaded, low-flow plumbing.

Kill approach:because it's a biofilm, it needs a different treatment than algae. Brush and remove the visible slime, then heavily chlorinate the whole system and run the pump continuously so sanitizer reaches the plumbing. A full system cleanout — filter clean plus, in bad cases, a chemical purge of the lines — is what stops it returning.

How to tell algae from just-cloudy water

Not every off-looking pool has algae. Cloudy water is usually a chemistry or filtration problem, not a living organism. Two quick checks tell them apart:

• Color and location. Uniform milky-white or hazy-blue haze throughout is cloudiness. A green, yellow, or dark tint — especially concentrated on walls, steps, or shaded corners — points to algae.
• Does it wipe and return? Rub a wall with your thumb. If a smear lifts and the spot re-colors later, that's algae. If the surface is clean and only the water is hazy, you're looking at cloudiness.

If it's haze rather than algae, the fix is different — see why is my pool cloudy before you dump algaecide into a pool that doesn't need it.

Why Florida pools bloom so fast

Algae is opportunistic, and Florida hands it every opportunity. When free chlorine hits zero in summer heat, algae can double roughly every 4–6 hours — how a clear pool goes green in a couple of days. Fertilizer runoff and heavy rain also load our pools with phosphates, algae's favorite food. Phosphates don't cause a bloom alone, but they let one explode once chlorine drops.

That combination is why Pinellas pools need tighter chemistry than pools up north. We cover the phosphate side in detail in our guide to pool phosphates in Florida.

How to keep algae from coming back

Every algae type above needs the same thing to take hold: a chlorine gap. Close it and you close the door. The prevention routine is short and it works:

• Hold a free chlorine residual of 2–3 ppm year-round — never let it drift to zero, especially before a storm or a busy weekend.
• Keep cyanuric acid (CYA) in range (30–50 ppm). Too much CYA locks up your chlorine and invites algae even when the numbers look fine.
• Keep phosphates low and rinse fertilizer overspray away from the deck.
• Brush weekly — especially shaded walls and corners — so nothing gets a foothold.
• Keep the filter healthy and run the pump long enough for a full daily turnover.

When to DIY and when to call a pro

DIY makes sensefor light-to-medium green algae where you can still see the bottom — balance, triple shock, brush, run the pump 24 hours, clean the filter. That sequence clears most green pools in a couple of days.

Call a profor mustard algae that survives two rounds, any black algae, pink slime or white water mold, or a pool so green you can't see a step. Those need sustained elevated chemistry, equipment decontamination, and repeat visits — and a badly overgrown pool is often faster and cheaper to hand off. That's where a one-time deep clean & green-to-clean or targeted algae treatment pays for itself.

Pool Optics is CPO-certified, fully insured, and month-to-month — the same tech every visit, with a photo report after each service so you can see exactly what the water is doing. We treat algae across St. Petersburg, all of Pinellas County, and west Hillsborough. If your pool has turned — any color — send a couple of photos through our quote form or call (352) 586-0364and we'll tell you which type it is and what it'll take to clear it.