In my experience, lionfish will eat anything they can fit into their mouth. I have a boxing crab in my current tank, and, in a previous tank, I used to have a volitan lionfish, so I don't have any experience with them together, but have observed both quite a bit. The boxing crab will stay hidden most of the time. It will scurry along the rocks, along the cracks looking for food, although it usually only comes out while I am feeding. The lion is an ambush predator. It will move slowly up behind something and snatch it. I think it would be difficult for him to sneak up on the boxing crab, since the boxing crab likes to stand in places with a rock directly behind him. With that said, i think the lionfish would eventually catch the little crab napping. It's a classic case of predator and prey. The boxing crab is a wary little crab, but it only takes 1 mistake.