Follow Jan's suggestions above. But, my guess is that it is just dirt. I vacuum my pool and it looks spotless. But a few hours later when there is nobody swimming and the water gets still, some of the dirt that was just in suspension in the water and thus not sucked up by the vacuum, falls back to the floor. My pad underneath my liner was taped together with some sort of tape and the dirt in my pool wants to settle in the lines created by the tape seams. It is no big deal and like Jan said, there is really nothing you can do about it. Unfortunately, there is no way to keep a pool perfectly spotless. Your best bet is to just go swimming a lot! When you are in the pool, it will get stirred up and you won't be able to see it!