Looks like an Arca sp. (the Turkey Wing Clams); there are a ton of species in the genus, so I'm not sure which one. It seems some species are facultatively mobile as adults, so it might be able to move a little bit on it's own since it's not attached to the rock at the moment. Based off of other Arca spp., though, I would guess this guy would prefer a rocky bottom it can attach to using byssal threads. It would probably also like having plenty of phyto (I'd recommend Isochrysis galbana/T-Iso and a Chaetoceros sp. or a blend containing one or both of these in particular if you want to try and add phyto to feed the clam). I'm not sure what flow these guys would prefer, but I'd guess a moderate flow would be best (enough to carry food for them to feed on, but not so much that it can't send out byssal threads).
