Clams really take there time getting used to a new tank, sometimes it can take a few weeks for them to fully adjust. Try putting a half shell or flat piece of rock on the bottom of your tank (under the sand), then place your clam, buried slightly, in the sand on top of that. That way when it does get comfortable it'll put its foot down on the half shell instead of the bottom glass or your rock work, and you can move it around safely. You really don't want to move it around much though, ESPECIALLY if its attached itself to something you can't move because you might damage its foot, and if you do that you could seriously harm the clam (just like an Anemone).
-Kat