Mustard algae needs aggressive shocking and brushing.

Shock the pool at triple the normal dose and brush every surface daily.

Mustard algae is a stubborn strain that looks like yellow dust on walls and floors. It’s chlorine-resistant, so a normal shock won’t cut it. You need to raise the free chlorine level to 15–20 ppm (check your CYA first) and hold it there for 24–48 hours. The brushing is critical—you have to physically break the biofilm so the chlorine can reach the algae.

Run the pump 24/7 and backwash or clean the filter regularly. Algaecide labeled for mustard algae can help, but it’s not a substitute for the shock and brush routine.

This stuff comes back if you slack off. Don’t let up until the yellow is gone for at least a week.