Sea Monkey Scuttle??

I have a tank of Sea Monkey's (brine shrimp) in my office at work and I watch them from time to time. I've noticed there seems to be a m of stuff on the bottom of the tank. It's a brown color and the Sea Monkeys seem to turn themselves upside down and fan it like they're sweeping the bottom. Just curious if anyone out there knows the what's or why's of this behavior. Any information on Sea Monkeys welcome! I love the little guys!