So I passed the OCLT night before last, but I still see algae on the floor of my pool. It won't scrub off with a nylon brush. Last night I got into the pool and with a butter knife scraped at some of the spots on the floor. They look black but when you get it to start coming up on the knife they are a very dark green that when smeared is a green slimy substance. So I've been told that I can't have black algae because I have a vinyl pool liner, but I can't figure out what else this may be. I've scrubbed and scrubbed for two weeks and although it's made some difference (and keeping the pool up above shock level) there are still some spots left. What are your suggestions?