I've seen that happen before... to my tomato clown! Your clown tried to eat the crab. Are you feeding enough?
When that happened to me, I netted them both and placed them in gauze soaked in their tank water. Then, I took a close look and found the clown impaled itself on one of the crab's legs, apparently when it bit down. Noting the angle of the crab's leg into the clown fish's mouth, I pulled gently at the same angle and it came right out. Both went right back into the tank and acted as if nothing happened.