Well, IMO, you prob. should have asked those questions before you got the clam...
Someone could prob. help you if they knew:
What kind is it?
What size tank do you have?
Whats your water levels?
I ASKED BUT ALL THE LFS TOLD ME WAS THAT I CAN HAVE ONE NO PROBLEM AND TO NOT LET IT OUT OF THE WATER NOT FOR ONE SECOND OR IT WILL DIE IN ABOUT THREE DAYS.
you dont have the lighting for a clam. you shoudl have much more.
a picture would help ID it. also, how old is your tank and how big is the clam?
also, you can let them out of the water, just make sure no air gets inside the cavity.