Get a tall drinking glass and prop it at a fairly steep angle against your rocks close to where you think he might be hiding. Put food in the bottom of the glass, preferably something nice and fleshy, and wait. He will crawl in the glass to get the food, but be unable to climb the glass to get back out. Do this at night, since that is when they come out most often looking for food.
You will gather every hermit, emerald crab and shrimp you have before you finally catch him, move them to your sump. He will eventually trap himself.
Support marine aquaculture, and share with your friends! Then you will find out who they really are.