Looks like some sort of bivalve (clam)? If so, these are filter feeding, and will eventually succumb due to lack of nutrients or phytoplankton.
They usually come as hitchhikers and eventually perish... where did you get the live-rock? I would remove the shell/carcass if you don't want the extra nutrient spike... or leave it if your system has enough livestock to quickly make a snack out of it!