Its a crocea. Mine loves the sand, and if its on the rocks it tosses itself off. Are you sure its the foot it lost and not the byssal threads? Google for a pic.
If you pulled the clam out and the foot ripped, the clam probably wont make it. If it ditched em itself, then no worries.
not sure what it is but if it was the foot would it still open back up right away?