Black Algae in a Fiberglass Pool 2015

I had some black spots show up in a fiberglass pool many months ago @ big around as a quarter with lines streaming from them forming an almost starburst look. I left them. Now, recently, over the past 3 weeks there are black spots showing up all around the pool that clearly look like black algae. Anyway to know for sure??? I read that black algae in a fiberglass pool almost never happens. If it's not black algae, anyone have any idea what it could be???? Thank you.