bivalves frequently show up on live rock. the origin of the live rock may help narrow it down some broad choices, but it could just as well be a common pan-oceanic species. there are thousands upon thousands of bivalve species. you're looking at a university-level specialist to ID that critter accurately if you are so inclined. ;Bookworm
he seems to be holding his own. you could pick up a bottle of Marine Snow or a Reef Nutrition product (Phyto Feast / Oyster Feast), and spot feed the critter with a 10 mL-size turkey baster ... see if it improves his chances of survival, but that might get cumbersome really quick doing it daily (or several times a day).
if you do give it a try, you’d want to shut down yr flow temporarily so the food suspends in the clam’s general vicinity.
Thats the thing I have never fed him. He must get alot of food otherwise he would of died. I dont know how long they live for either. But on forums nobody can keep them alive for long. Hes actually in a low flow area aswell. Ive had him years now I just just dont know what type is. A clam or Mussel.
I think he is some kind of mussel, He seems to be very flat and his shell is long and not round. Sorry for the quality but i took these when he was closed on my nokia 6.1.