Tunicates/ odd sponge encrustation?

[Marc G-L]

Saw this when I was cleaning GHA outbreak in my tank. Attached to a shell that was shielded from lights under my neg space arrangements.

Any clues?

 

[Tahoe61]

Are there Tridacna in the tank? If so that looks like byssal threads.

 

[Marc G-L]

No clams, I did find two 3/8" silvery tear drop shaped oysters in my fuge. If you expand the pics the lower side of the shell seems to have small white colonies of something. They don't sway in the current. So, I'm still thinking tunicates.