It doesn't look like "black" algae, so I think the black stain is most likely from the rebar "rusting" below the plaster surface (gunite) and then bleeding to the surface. To fix requires chipping out the plaster and gunite down to the rebar and removing and treating properly so that it doesn't rust again. Chlorine tabs, acid, and other treatments won't stop this type of problem because it is all the way through the plaster coat.