Crab in my Brain? WARNING HUGE PICTURE!

[Rippin]

..coral, that is.

I have a Lobophyllia and I recently spotted what I think is a crab embedded in the folds of the brain coral. The claws are VERY small compared to the body size. When it starts to creep out it definitely tears the brain's flesh, but so far, the area doesn't seem to be dying or diseased. My question is: is this crab symbiotic, a pest, harmful or harmless?

In the picture below, it's the largest 'hole' at the bottom right corner. (I told you the picture was big!) You can see the creature's head and one leg.