Not what I thought

[mah0031]

What I thought was a shell stuck on my live rock I bought about a year ago opens and closes about an 1/8 of an inch. Shocked the hell out of me, what is it?
These little

white lines on my rock are I assume small tube worms good or bad?

 

[Mike_J]

The feather dusters are good filter feeders

 

[KJ]

Pic 1 Bivalve Clam, Oyster, Musse.