Okay, I tried scraping the spots with a trichlor puck, and with TONS of scrubbing, they do come off. The issue now is that the spots have multiplied over the winter (I probably have 100 spots now), and scrubbing each little spot with a trichlor puck at 2-3 minutes each is going to take me all summer. Any other abrasives I could safely use on fiberglass pool steps? (There are no spots on the sides/bottom, thankfully.) Is a copper scouring pad okay? What about a pressure washer? Thanks for the ideas!