I happened to be standing next to the tank with the camera in my hand just in time to catch the cleaner shrimp cleaning Clack.
He cleans the tang daily, and Clack often, though Click seems more skittish. You can see he actually climbs on while Clack stays close to the rock, and then goes back and forth. He jumps on and off a few times, the whole thing is such a dance and both the shrimp and the fish seem ready to bolt at the slightest provocation.