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.

I see what looks like two different stains, dark spots and more smudged gray areas. The dots look somewhat like black algae, but the gray looks more like metal stains.

I suggest you try holding a vitamin C tablet against a stain/algae spot for 30 seconds, and you rub another spot/stain with a trichlor tablet for about a minute and see what effect those two treatments have.

Scrape the spot with your fingernail and rub the scraping on the back of a white business card - if the smudge is green - it's black algae and treat the pool appropriately.