I drained about 1/4 to 1/3 of my pool two nights ago. I promptly refilled with clean water within 15 hours after that. The stairs that were exposed to air during the draining process now have brown spots on them. If I rub my fingernails hard on the spots, they seem to come off. The spots are not easily removed; they seem like pieces of metal. What are they and how do I remove them?? I've also attached pictures of a napkin that I stuck directly into the pool and rubbed onto the spots.
Thanks and have a great day.