What is growing on this rock?

[Halal Hotdog]

Was at the LFS the other day and saw this growing on some of their live rock for sale area. So it is calcified (even though it looks like a cucumber), with an inner hollow area where I presume something lives. Found it to be very interesting but no idea what it is.

 

[willow659]

It’s a tunicate.

 

[Halal Hotdog]

Tunicates I have had looked dramatically different, but after a few minutes on Google I quickly realized you are correct.