I was reading a crab expert (meaning they were a prof whose primary research had to do with crabs) posting in an ID forum before and they said the vast majority of crabs are not reef safe and that, in general, it is safest to get them out when you have a chance. The advice was based on the fact that crabs tend to be opportunistic and even those that don't eat coral can usually get big enough to pose a danger to snails, small fish, etc.
When it comes to crabs I follow one rule. If it isn't clearly an acro crab or I didn't purposely put it in there, I kill or sump it ASAP. They can be so hard to get I don't hesitate. When I was accumulating live rock I got crabs in my tank a couple of times. One crab took me months to finally "remove." Since you've now seen him munching... good luck getting him.