Depending on the type of pool you have, vinyl/fiberglass or gunite, the cleaning will vary slightly. For pool openings, liquid chlorine or acid can be poured down the tile/gunite surface to help remove surface staining. Brushing vigorously, using a magic eraser may also help or a pool pumice stone, just test on a small section first to make sure it won’t cause discoloring or scratching. There are also chemical additives that can be added to the water (and then raise the water level to hit all the affected areas).