Looks like a decorator crab to me. I just got rid of mine because I caught him eating my acans at night. They're kinda slow and clumsy and very nocturnal. Their body is an orange color but they'll pluck off your soft corals and put them on its back to attempt to camouflage. Does he have zoanthids on his back?
Pistol shrimp are very quick and agile (you would be lucky to get a picture of one) and stay on the bottom of your tank in the sand, and you will see little piles of sand from burrows it creates with entrances under the edges of your rock.
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