I have something like this too- they are all white and hide in cracks and under stuff... I see there legs sticking out of cracks everywhere, my reef tank (30 gal) is THRIVING and all the corals I put in spread like wild fire... nice tank but I have hundreds of these things!!! Maybe over a thousand, if I stir the sand bed they go floating through the current (they roll into a ball when this happens) all over the place, anyways- my tank is thriving with no probles ever with pollution or anything so I just leave these guys do their thing... Mine seem to max out at about the size of a quarter spread out, most are size of a dime spread out and some smaller