Mustard algae is stubborn – raise chlorine and brush hard.

Yes, if it looks like yellow dust clinging to your pool walls and comes back within a few hours of brushing, that’s mustard (yellow) algae. It’s more chlorine-resistant than green algae and loves to hide in shady spots, behind ladders, and in your filter.

Treat it like a pest that needs an all-out assault. First, brush every surface – walls, floor, steps, behind lights. Then shock the pool to a much higher chlorine level than usual, like 15–20 ppm (check your shock instructions). Run the filter 24/7 and backwash or clean it daily. You might need a specific mustard algae algaecide if it keeps coming back.

Also, wash any swimsuits, toys, or floats that could reintroduce it. That algae hitches rides on gear.

You’ll beat it, but it’ll take a few days of brushing and shocking. Stick with it – the pool will turn clear again.