It's not gone. Once you have black algae, it's rarely gone, unless you've drained, pressure washed, and resurfaced. It gets in, and under the plaster (see picture series below). You can kill the bits that are exposed, but not the bits that aren't.

You have to maintain levels of chlorine that prohibit further growth, because it tends to re-emerge the first time you give it a chance.



3/4" glass tiles to the right.


3/4" glass tiles above