Sand all on my rocks?

Gerbo

So every morning when I wake up, there is more and more sand on my rocks? It is like sprinkled over. I think it's one of my fish. I have 2 clowns, a banggai cardinal, and a mandarin (and a peppermint shrimp). Which one would it be and what's it trying to do?

Reefing Madness

Clown fish, diggin in for the night. They will make little dens by rocks, using their tails to clear the area out.