can anyone tell me what this is??

[riatm]

This is attached to a rock that I bought with some mushrooms. It looks kind of like a worm. My spiny puffer just went after it and it retracted back into this shell.

 

[riatm]

maybe you can see it better here:

http://s318.photobucket.com/albums/mm432/RiaTM/?action=view&current=IMG_2587.jpg

 

[greenbean36191]

It's a vermetid snail. They're relatively harmless, but their snot webs can irritate some corals and most hobbyists find them an eyesore. You can crush them or superglue over the opening of the shell to kill them.

 

[4218kris]

aka tube snail

 

[finsurgeon]

If you look for the threads on vermitid snails, you'll find a lot of people who deeply regret not smashing their first one when they had the chance. Do it now, before it's too late. They will spread and they will clear out an area of corals.