Mollusk and aiptasia?

[KayKaytn]

Hi everyone. Used live rock to start aquarium. It's been over 3 weeks and I am starting to see new hitchhikers show up. I have two in the same place. The first muscle-looking thing, I have no idea what it is. It is the shape of a muscle, but the shell seems quite textured. I tried to remove it from the rock and it seems to be attached to it quite firmly. It also seemed to leek some whiteish stuff when I tried to remove it. The second is what I suspect is an aiptasia but it's so small and flat on the rock (I can't see a trunk; it seems the tentacles are growing from the rock). I've looked at so many photos of aiptasia, but I am just not sure. The last is what I think is a sponge.

 

[Crabs McJones]

Aptasia and possibly a bivalve. Not sure the last picture

 

[cracker]

looks like both to me .

 

[KayKaytn]

Thank for the responses. I, fortunately, looked under that rock and saw that it was covered in them. I hadn't seen them as they've been hanging out under there. I actually just removed the entire rock. I just hope they didn't release any of the basal cells. It's a pity because it was my favorite, but I'd rather replace it with a dry rock than battle with an infestation.