Just a suggestion... it may be sand rather than salt. When I added salt to my pool (4 bags) there was residue like that left after the salt dissolved. It's either sand or some kind of crushed rock--I think. Anyway, I'm pretty sure it's harmless and not chemically reactive, so I just vacuumed it up into my sand filter system. Problem gone! If it is a salt compound, It should eventually dissolve. If rock or sand, then the filter is where it stays.