So commensal shrimps and clownfish aren't the only things that live in anemones. This is an anemone crab or porcelain crab.
These aren't true crabs so much as relations of squat lobsters. Not visible here is its feeding apparatus which is basically a huge fan like baseball mitt thing it uses to catch passing microscopic planktonic food. Apparently some other species don't use anemones prefering to ride shotgun with hermit crabs in their shells.