What kind of snail is this?

[Paxthemax]

I found a tube on an old snail shell. I thought it was a slug but it has a hard shell, the hard disk and the mouth moves like a snail's.

The next obvious question is: good or bad?

Thank you for the help

 

[Crabs McJones]

I found a tube on an old snail shell. I thought it was a slug but it has a hard shell, the hard disk and the mouth moves like a snail's.

The next obvious question is: good or bad?

Thank you for the help

Either feather duster worm, or a vermedit snail.

 

[KJ]

Vermetid snail

 

[Oldsalt01]

Yep. Vermatid. You can see the mucous "fishing line" they extrude to pick up detritus and plankton to feed on. There is anecdotal evidence these lines can irritate corals.

 

[Oldsalt01]

Some say they can take over a tank, but I had two on one of my Trochus Snail shells for 2 years and they never did spread.

 

[Paxthemax]

Thank you everyone.