Tube worm looking thing

[Mschmidt]

Sorry for the bad picture, the sucker is fast to retract into a white stony tube. It can be seen on the right side of the favia at the top. I suspect it's the reason the zoa next to it is closed. What is it and what should I do about it? Also I know the algae isn't helping...

 

[Jekyl]

Aiptasia by the looks

 

[Mschmidt]

Kinda looks that way, but I have never known aiptasia to have a calcified shell. All that white under it is its shell.

 

[WheatToast]

Some feather duster/tube/fan worms grow these calcified shells. Harmless filter-feeders:

 

[Mschmidt]

Some feather duster/tube/fan worms grow these calcified shells. Harmless filter-feeders:

That's pretty close to it. thanks!