Black algae
Also called: cyanobacteria, blue-green algae.
Black algae is not algae but cyanobacteria, a bacterium that glues itself into the pores and micro-cracks of plaster, gunite and concrete under a waxy cap chlorine can't penetrate. It shows as dark, raised spots that regrow in the same places after brushing, and no bottle kills it.
Why it matters
Confirm it first: scrape a spot with a fingernail and smudge it on white paper; a greenish streak means black algae. Then break every spot open, spot-dose with strong liquid chlorine, hold free chlorine at about 20% of CYA for 2–4 weeks, and brush the spots daily, because the cap reforms quickly. It is almost always a sign of chronic under-chlorination. Skip the "black algae" bottles and never use copper algaecide. On a vinyl liner or fiberglass it is rare, so doubt the diagnosis, and never put a wire brush or a pressure washer on a liner; nylon is the bristle that is safe there.
In numbers
Trouble Free Pool's example: at CYA 70, keep free chlorine near 14 ppm for the 2–4 week hold. Afterwards the floor is 12–15% of CYA for the life of the plaster, so at CYA 50 never below 6–7.5 ppm.