Any clue to what this is

[fuzznut]

Got this growing on my shell.

 

[jraker]

vermetid snail?

 

[sam.basye]

+1
They often release a long stand of a mucus-like net that waves in the current from the tube opening.

 

[nickman]

Definitely a big vermitid snail. I am a huge proponent of biodiversity in a reef system but will kill those without hesitation. I have had one too many of their tubes stuck in my hand while moving rock around.