Brush and balance.

That’s it. Black algae on plaster surfaces—especially in pools—is tough because it forms a protective layer that regular chlorine struggles to penetrate. The fix is not complicated: scrub the plaster with a stiff brush (nylon or stainless steel, depending on your finish) every few days, and keep your water chemistry in the right range—specifically chlorine and pH. If you let the algae get a foothold, you’ll need to shock and scrub repeatedly. Prevention is way easier than cure.

A phosphate remover can help if your phosphate levels are high, but that’s secondary. The real trick is not giving the algae a chance to establish. Regular brushing breaks up the biofilm before it hardens.

This is not a set-it-and-forget-it situation. Plaster needs attention.