Did it react at all? Sometimes in LFS's the light or water quality is not good for these gorgeous clams. If you have a good system and good lighting he should pull through.
You can try moving your hand in the direct path of the light and casting a shadow over the clam, does he react by retracting his mantles?
Also, you might want to keep in mind that depending upon the light the LFS had him in "some will hold them under vho's" and if you have MH then you might want to put him low and out of directly light and then slowly move him into the direct path of the light and up more and more.