Back in 2006, I noticed this Pavona coral had a tiny dweller. She didn't come out and crawl around; during the daytime she withdrew more deeply into the coral's protection. It's called a Gall crab, because it creates a gall (or cave) in the coral it hosts within. I never saw any reason to be concerned by this crab, primarily since it never left its home. It captured any food within reach. According to one source, the female is trapped in the gall while the male can come and go as he pleases. They breed in their cave where the female protects and raises their young.
Here's the coral, and the arrow is the spot where I discovered the lil crab. That entire shadowy spot is not its cave. Just a tiny segment of it is. I enjoy looking at my reef a night using a flashlight. I carefully study corals and the tiny stuff that zips about, but coming across this little guest was a surprise.
Additional reading: http://www.dafni.com/crustacea/hapalocarcinidae.htm