Help! What is this?

[Amandaew]

I have two of these in my tank. Unfortunetly, I was unable to upload a video I had taken but, I’ll try to explain it best I can.

a dark mouth comes out of the white and casts a web. It looks like it “reels” in the web with its mouth. I’m assuming it does this to catch food in the water. The white seems to be just a shell. It never moves and seems like it’s the same size. Although, it was not there when I first got my live rock. Any ideas on what it could be?

[Nocturnal]

From the description it sounds like a vermetid snail but the pic doesn't look right. 

[sublunary]

Yep, vermetid snail. The orange windy tube towards the bottom of the pic. 

[ihglifelol]

those are fine, unless you have a sensitive coral next to it.

in one on my tanks, it lives on my acan, and the acan just steals its catches of leftover food lol.

[mndfreeze]

Yeah, I have a bunch in my tank.  They do spread if there is enough food n stuff.  I only actively kill em if they are in a spot that is irritating a coral.    I have one under my hammer and the hammer gives no fscks.  I have seen zoa's get annoyed by them.   Just super glue its tube opening shut if you need to kill one.  Otherwise they just help clean up more detritus.