who made the mess?

[MaryPa]

Something is putting sand on top of my live rock ( 4-5" high ) .I woke up the last 3 days and found it like that.What can be doing this?

Occupants:4 redlegged hermit crabs 1/4" big,2 yellow finned blue Damsels,a firefish goby and a turbo snail.

 

[VDub90]

My yellow tail damsels love throwing sand around. They make little caves for themselves under the rock, they usually pick it up with their mouth, and sometimes go right to the top of the tank before spittting it out. So this causes a little sand everywhere.