I made mine a pseudo cave with the way I aquascaped, and he took rubble and shells and made it more to his liking. I had some SPS in the tank and some encrusting corals that were above and around his hole that eventually grew in quite well.
Not sure if he actively "stopped" the coral from spreading over or into his burrow, but it encrusted to a near perfect circle around the entrance and never went farther in even though light penetrated another half inch. You could also definitely hear him chipping away at pieces inside.