Salt on bottom of pool?

Hello everyone. I have an above ground resin 15x26 oval pool with a compupools saltwater system. I have a Hayward sand filter and hayward pump. I put a solar blanket on at night to help hold the heat. I run the pump 24/7. My question is why it looks like I still have salt that is not absorbed into the water on the bottom of my pool. Pool was new 3 weeks ago. According to the system salt level is fine and I have it set for 30% and there is 3500 ppm salt. When I tested the water the free chlorine is a little low. Any ideas on what I can look for. Thanks.

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.