That clam is not dead, just unhappy! It could be jumping off the rock because it doesn't like the flow, or it's not getting enough light, or something (like a fireworm) could be bothering it from within the LR. Another Idea I got from a local reefer is to place them on a small piece of ceramic tile. The whole goal in placing them on any surface is to give it a place to attach itself where it will be protected from predation. Consider placing it on a tile and placing that on the sand in direct light. You can build a "campfire" with LR rubble to keep it upright for a week or two while it attaches.
