Porcelain anemone crab. They are actually white but it looks like he has algae growing on him. They form a symbiotic relationship with anemones. One of the most peaceful crabs I know. Heres a pic of mine
How much was that lfs asking? I wouldn't pay more than $15. I got mine for $8. And i know they are partial filter feeders, its cool seeing them fan the water lol