You say you doubt the little critters be actually in contact with flesh of coral? Where top of hole is with critter that cant yet see in biggest hole is like 1 cm under flesh of coral when looking from outside so if you right in saying this,in theory i could get a bandsaw or possible dremel or a sharp hand saw and cut off the skeloton above where i see the hole,? Then epoxy/glue it back onto rocks.I would say that they are most likely both some type of amphipod. They may in fact be two separate species considering one of them is at least somewhat motile while the other appears to stay relatively stationary. To the best of my knowledge, the vast majority of amphipods are not harmful and likely have just moved into those existing holes near the base of the coral. I would be surprised if they are in contact with the actual flesh of the coral at all considering the location of the holes. The more motile ones are likely feeding primarily on copepods. While the less motile is just grabbing whatever floats by copepods detritus algae etc...
This could work aslong as it isnt a critter that burrows through the skeloton and there is none anywhere else in tank.
Just a thought