what is this

you might need more flow, a bigger clean up crew, or both.

 

some different kinds of crabs tend to eat detritus maybe try adding a sally light foot or emerald crab

 

do you have a skimmer, if so what kind and if not it would be a good idea to get one because when you have a skimmer it "produces air bubbles which the detritus adheres to, and forces it outside the tank before it decomposes" (wikipedia). just an idea

 

ok i show the pic to my marine store they said its normal cause of all the sand blowing eveywhere and the blue shrek goby making a mess with making a home i got a clean up crew and some carbon for my water cause its gone musky from the sand dust