That's a synalpheus pistol by the looks of him. Those little guys are amazing, and various types of them can be found living commensally with just about any kind of 'host organism.'
So long as he seems to be pinned to that area, and you're not worried about any critters messing with him, he should be a great addition to your setup! I have somewhere in the neighborhood of six, all different and all hitch hikers on various rocks, squirts and gorgos.