ID Hitchhiker Please!

[Goldie-Maree]

Does anyone know what type of mussel/oyster thing this is? Might I add it's wedged in and not mobile. Very much alive though.

 

[cloak]

I don't really have a positive ID for you, but perhaps this link will help.

http://www.reefkeeping.com/issues/2005-04/rs/

 

[bobman]

Most likely a Mytilid clam. Harmless filter feeder