This came into my tank by stowing away on a cluster of zoas. I saw it attached in the cluster. Its about 25mm x 10mm. I had tried to gently pull it off, but that didn't work, and the outer material/skin/shell was pretty hard so I just left it alone. This was weeks ago. Yesterday I see it has migrated and its outer layer is starting to grow some of my newly budding coralline algae. The odd thing about the migration is that it went from the highest peak to the 2nd highest peak of my live rock. So this thing climbed or swam... It looks clam- or muscle-like to me. Can they climb or swim? I really have no idea. Do you have any idea what it is?