Tundra,

I'm sure you'll get lots of good help here.

First confirm that you've got algae. Scoop up some of the gunk and rub it between your finger and thumb. If it's gritty, it really is sand.....probably not. It'll probably be pretty slippery (and slimey) but let's confirm that first.

Second, please post current test results for FC, CC, ph, Alk, and CYA....that'll help us all understand better what you've got going on and advise you of a path to get it fixed.