Hi folks. I live at Amelia Island, just outside Jacksonville FL, and my pool is just meters away from the salt marsh. Pool is in-ground 15,000 gallons with salt water/chlorine generator. I test it every week and keep chlorine, Ph and alkalinity levels where they're supposed to be. I don't have numbers to give, but use the dye-based system to stay in the right color range. Anyway, this summer I noted in just a few places dark spots, about the width of a pencil eraser. They wouldn't vacuum up, so I tried to brush, and they were very resistant to that. Finally I just got in the water and scraped a couple off with my fingernails, and it was a dark green, fibrous material that looked a lot like seaweed. I've read about mustard algae but the pictures I've seen on-line don't seem to represent this. The pool is otherwise nice and clear and as I said, the basic chemicals always test fine. Any thoughts about what this is, and how to treat? Thanks!