Well, I have no idea what he is but I'll tell you what I do know...
He is extremely shy, I had him for at least a year before my 10 year old spotted him. He eats coralline algae, as shown in the pics. You will see scratches on it in the morning. He will also eat meaty foods that you put near his hole. I feed him silversides. When he molts he will blockade himself into a hole, don't worry he'll manage to get free on his own.
He has never harmed a fish, never harmed a coral, never harmed a shrimp or anything else for that matter. Had him since tank set up with no probs in a full reef.
Never did find an id on the little guy.