Sounds like you have mustard algae. Its brown and looks like sand. Sits on the bottom of the pool in spots about htes size of a quarter. Sound like your problem? I don't think its sand since you stated that it disappears when touched. Sand doesn't dissolve, so it wouldn't do that. This algae seems bloom right after a good rain. You need to shock like they say here and hold it there for a couple of days or even a week until the algae doesn't return anymore. Also need to vacuum that stuff to waste to get it out of your pool. To keep it from coming back in the future, I would shock every time after a good rainfall.