I have an in-ground salt water pool, 20x40 and I have to admit that due to school and working full time all summer I didn't have the time to tend to it daily like I should have. For the last part of the summer I've had to vaccum daily but about an hour after vacuuming the dirt like stuff would re-appear, so I'm really fed up right now. I want to close the pool for the winter but I need to make sure this is taken care of before I do it. First off, this guy that works for a pool company was unable to identify what it is. The thinks it's dirt. He claims it's not black algae but I don't think he knows what he is talking about. When I vacuum it, it disappears into fine mist. Is this black algae? Please help because i'm extremely frustrated! I've vacummed with both a pool blaster which is separate from the filter and I've also vacuumed through the filter/skimmer thru waste.