bill
many of these clams are subject to open air during low tides. Like other mollusks, they will "clam up"
and maintain their fluid levels in their shells until high tide. so taking them out of the water for few minutes to an hour should be fine.
the lack of a foot is concerning- most healthy croceas should have a foot of some sort and they secret strands to the ground to attach themselves firmly.
use a magnifying glass and small tweezers to hunt the small snails down.