I am going through the same issue. I have been fighting this battle for 3 years now. Everyday I vac the pool and by the next morning I have the spots in the depressions of the liner. They look greenish in color. They vac easily and will dissipate if I brush them. I keep my levels right where they should be. I have done the overnight chlorine test and that too is fine. I added sand to my filter this year to bring up to the proper level as all the backwashing last year I lost a bunch. I was hoping this would help but it has not. I am at a loss as to what this stuff forming on the bottom of the pool could be. This is very frustrating as all my test and levels are indicating there is no algae in the pool but these spots show up every morning.