[mps9506]

Weird, looks like an air bubble attached to the top of it. I haven't had to deal with Vermetid snail or a Spionid worm so I would follow Charles' advice. I do agree that the coral should not have any issues growing over it once it is dead.
It almost looks like dinoflagellate that had attached to an outgrowth of the skeleton, but in that case the bubble would just come off once you touched it.