corcea clam concern

Bought a clam on the weekend and put it up in my rockwork and came home yesterday to find it laying upside down in the sand with my seaweed blenny burrowing around it. Rescued the clam but to my alarm could not see it's foot(retracted) if they can do this. Not sure if my seaweed blenny made a snack out of it or not. Anyways I placed a flat rock under the sand and placed the clam there last night. Since this the clam has not fully opened up. It is reacting to shade etc by retracting what little of it's mantle is out in. I think this is a good sign. What concerns me is I came home today and the clam was in a totally new position in the sand bed(facing right side up) so I moved it over to where i buried the flat rock and had a quick look and still see no foot out. Anyways just looking for suggestions or advise if this clam will survive or not. Thanks in advance