This little critter came in on the bottom side of a new zoanthid's little chunk of live rock I think. It appears to have a dark, leathery covering over a pearlescent shell that it can stretch to either cover it almost completely or expose much of it. The covering has some tentacle looking projections.
Any clue as to what it is so I can learn more?
Any clue as to what it is so I can learn more?

