Help with dead algae??

Had a problem in my pool all last season. I would vacuum pool and few hours later I would see debris all over floor again collecting in little clumps. When I would hit it with the vacuum it would puff into a cloud. In spring I had a new liner put in pool....by the second month I had the same problem in my pool. I couldnt figure it out. Thought it must be dust from the lawn mower getting into my pool. It was there every day and as much as I vacuumed I couldnt get rid of it. Friend finally convinced me it's dead algae that must have been living in the bag of sand used to weigh down my drop in stairs. So, now what? Help me get rid of this awful problem please. Pool water is always crystal clear although I did have some yellow algae collecting behind stairs and I put in some Mustard algae killer.