Mustard Algae: The Chlorine-Resistant Strain (and How to Beat It)

Mustard algae — also called yellow algae — is the most stubborn algae species Florida pool owners encounter. It looks like pollen or sand on the pool walls and floor, clings to shady spots, and resists normal chlorine levels. Clearing it requires a sustained, aggressive treatment.

What mustard algae is

Phaeophyta (yellow-green algae) is chlorine-tolerant. Unlike green algae that floats and turns water cloudy, mustard algae attaches to surfaces — walls, steps, light niches, ladders — and forms a thin, yellowish film that brushes off easily but returns fast. It can also live on pool equipment and accessories brought from other pools.

Why normal chlorine doesn't work

Mustard algae has a protective mucilaginous coating that shields it from chlorine at normal levels (1–3 ppm). You need to drive free chlorine to 15–20 ppmand maintain it long enough to penetrate the biofilm — that's 3–5× what green algae requires.

Treatment protocol

1. Brush all surfaces thoroughly— scrub walls, floor, steps, light niches, and ladders. This breaks the algae's protective film.
2. Test and balance water — target pH 7.2 (lower end increases chlorine efficacy), TA 80–100, CYA at 20–40 ppm for treatment (lower CYA means chlorine works harder).
3. Shock heavily — add cal-hypo or liquid chlorine to reach 15–20 ppm FC. For 15,000 gallons this means roughly 3 lbs cal-hypo (65%) or 3 gallons liquid chlorine.
4. Add a mustard algaecide — "Yellow Out" or similar sodium bromide + shock combo products work well. Follow product directions exactly.
5. Run pump and brush again — brush every few hours while chlorine is elevated. Run filter continuously.
6. Clean filter aggressively — backwash or clean after 24 hours. Mustard algae clogs filter media fast.
7. Treat all equipment — soak pool toys, float noodles, and cleaning equipment in high-chlorine water or diluted bleach. Mustard algae re-introduces itself through contaminated gear.

After treatment

Once water clears, maintain FC at 5–7 ppm for two weeks to ensure complete kill. Consider a weekly algaecide dose of a non-foaming, non-quat product as maintenance if mustard algae has been a recurring issue in your pool.

Mustard algae always comes back when people under-treat. Don't stop at the first clear day — run elevated chlorine for a full 72 hours and deep-clean the filter before declaring victory.