My plaster is less than 1 year old. I have been keeping the FC at about a 15 level - (30 CYA) - for about 4 days now - and brushing pretty frequently.
These are pics right after having brushed the spots with an algae brush. I have seen some of the little black heads that people talk about. And I have knocked some of them off with the brush. I think this is black algae but what I am unsure about is the almost orange-ish color of the plaster underneath the location of the black dots.