Resurrecting an old thread here...
I had a similar or identical crab in my little 24 gal reef. I rarely saw him... I named him Trogdor since I really just saw one beefy claw at the bottom of a crevice.
Since I had a problem with vanishing zoathids I suspected Trogdor of eating them. I decided to get rid of him, and the zoa bait I put in the tank has remained unmolested.
I felt bad about getting rid of Trogdor but I am pretty sure he ate about $50 of zoas.