dom there are two that look similar one isnt bad the other is. does it look like this ? or is the body more textured ? check these out http://www.wetwebmedia.com/swcrabidfaqs.htm
They usually get the food they need when they are smaller off the algae on the rocks. when they get bigger they tend to go for corals to meet their food demands. Its when you see zoa's dissapearing that your gonna yell at your octo for not doing its job.
My pictures suck Jim (sorry). The crabs shell is very smooth. It looks very similar to a gorilla crab with no hair and colored pink. It has doubled its size since I put it in the tank. I didn't see it for a while so I thought the Octo ate him like the others.
I don't have many zoos in my tank. I do have one large colony of a brown zoo in the back of the tank that I wouldn't mind it eating. Right now it comes out to grab mysis shrimp. I'm assuming the crab has a nice stronghold in the cave but if it ventures out too far, the Octo is going to eat him.