I believe the eel will not be a threat to your clams. When they get big enough, they will be able to rearrange your rock to their liking. Then the avalanche could pose a theat.
I wouldn't recommend any angel in a reef tank. 50/50 chance they will nip on your clams, coral, rock, glass, thermometer, and any thing else in your tank. Some people love angel so much that they think it is worth the risk. Just keep in mind, how will you get it out if it turns out to be a problem?
Louey