The light you got is pretty low for a clam. Place him as a high as possible. Yes, you do turn off the lights. He fell off the rock because you didn't place him well enough. You should feeed the clam phyto plankton. You really should have done your research before you bought the clam, I wouldn't be suprised if he died.....If your clam is not sensitive to movement or sound, then that was an unwise choice, they should be alert. If the clam does not have a disc on its bottom, then its been polled from the reef, and it will slowly die. It is possible to get one from the reef with a disc, just if it was pulled off when collecting, he will slowly die. I would suggest never to buy anything unless you have extensive knowledge of what your looking to purchase.