Red Leg hermit crab
This is a Red Legged Hermit crab. It was trying to right itself in this image, and almost pulled itself completely out of its shell. Please do not confuse this one with a Scarlet Hermit crab, which is vivid red. The red-legged ones are considered safer than blue legged hermits. Hermit crabs are known to kill snails to get their shells, so I'll label this one Reef-safe-ish. They are a good member of your clean up crew.