mollusc

Hi all Tank is swarming with copepods and this weekend I finally get to add a fish (still deciding on the first tank mate!) I saw this weird clam like shell on the live rock. Any ideas what this is? I did think it was a clam or some kind of mollusc but kind of can’t believe that! Any ideas are...

Found this in my 6 month old nano tank. Live rock was from TBS (so Gulf of Mexico). It either crawled or grew over one branch of a gorgonian - you can see a gorgonian polyp at the bottom of the picture. It also has thin projections coming out from under the shell (not clear in the picture...