I bet it is just dirt. My pool does the same thing. (I don't have wrinkles in the bottom of my pool floor but there are slight indention lines where the pad beneath the liner is taped together.) When you vacuum, no matter how slow you go, some of it gets disturbed into suspension in the water. Then, overnight when the water quiets down, the dirt settles back out on the floor. It is no big deal because as soon as people are in the pool, it gets mixed in again and you can't see it. It is not possible to have an outdoor pool with absolutely no dust/dirt in it.