could be stain from metals in water starting or black algae starting.
if you have a plaster pool you can take a tab and rub it on the stain and see if it starts to come off, if yes black algae which can be taken off with different black algae treatments depending on what color plaster you have as some can stain colored plaster as they sit on the stains to remove them.
If the tab does nothing then its metals in the water along with PH probably being to high. You need to use a sequestering agent/metal remover that comes in 1qt bottles or 1/2 gallon and add to your pool along with lowering your PH level to 6.5-6.7 and scrub your pool a few times a day until gone. Then readjust your PH level back to 7.2-7.4.
You could also get a PVC pipe with a fitting at the end that could trap muriatic acid in it, so you can hold this over the spot and pour the acid into the PVC pipe which will sink to the bottom where the stain is. This would be like acidwashing this small area removing the stain with some brushing after and readjusting PH after depending on how much was needed.
This is for only plaster, pebble-tech pools.