I have had this pool for about 7 summers now. For the last 4 summers, I have had what was once thought to be mustard algae. I have spent hundreds on chemicals to get rid of whatever it is and still I have a problem. Every time I turn my filter on, I end up with what looks like sand pockets all over the bottom of the pool. When you look close at it, it looks kind of slimy. In 2010, I replaced my Hayward sand filter - thinking it might be sand in the pool from the filter since it only happens when I turn the filter on. That did not help. I replaced the sand once and it did not help, so we changed it to zeobrite and that is what is in there now. I replaced the Hayward pump at the suggestion of the pool store, that did not help. It was also suggestion, because of the sliminess, that it was a water mold coming from under the liner. There were some patches to the liner so... I replaced the liner at the end of last season. When I took the winter pool cover off this summer, the water was sparkling blue with just a little debris on the bottom. As soon as I turned the filter on, the problem was back. No one has any idea what this is. I have brought this problem to many pool stores and no one knows why it is only when I turn the filter on. Last week, I did not turn the pump on at all - we all played, made whirlpools, cleaned it with a leaf net to get all the bugs and debris out and the pool was beautiful and clean. I had to turn the pump/filter on to put the weekly chemicals in and within hours the bottom was covered with pockets of sandy/slimy light brown stuff. My pool is 28ft round and 4' deep - so it was a lot.
Does anyone know what this could be?? Please ask questions if you need more info, I don't know what info is pertinent in figuring this out. I do have trees around the pool, but any branches have been cut away from hanging over the pool and when the filter is off, there is nothing on the bottom of the pool. We have hosta plants around the pool The pool gets a lot of sun all day.