Sometimes mine does that after the kids have been swimming if there's any dust/dirt/pollen/sand in the bottom of the pool. It gets stirred up during the swim sessions. Some of it will get caught by the filter, most of it just settles back down into the seams of my liner overnight while the pool is off. If that's the case in your pool, let it settle overnight, then vac it out. If it doesn't bother you, just accept it as fact and don't worry about it If it looks at all greenish, or if you lose any chlorine at night (test at night and again in the morning before sun is on the pool), then just shock it one more time and it should finish clearing it up.