Clam hitchiker??

[authentic]

I've had this poor right rock for at least a year and a 1/2 and just noticed this growing on the side. Must have been getting bigger over time ID?

 

[Crabs McJones]

Bivalve. Really cool filter feeder

 

[authentic]

Must have been really small when it came in..one of the only pieces of live rock in my tank,,,xmas tree worm rock

 

[Wolf89]

Looks closer to a mussel than a clam