Can you get rid of mustard algae in a pool?

Yes, but it takes persistence and high chlorine. Mustard algae is chlorine-resistant, so you need to brush every surface, then shock the pool to 20-30 ppm free chlorine for 24-48 hours while running the pump nonstop. Brush daily, add a yellow-algae-specific algaecide if it returns, and clean your filter often. Don’t under-shock or skip brushing — that’s the common mistake.