ID help

[brianr24]

Pictures might not be best. But best I could get. It's on back of crab shell it looks kinda like my sub straight stuck to him. I think it's something weird tho.http://www.reefcentral.com/forums/attachment.php?attachmentid=348204&stc=1&d=1460840115

 

[smeesh726]

Can you post a better picture? Is it white?

 

[cloak]

Check this out. (Sabellaridae)

http://reefkeeping.com/issues/2003-05/rs/

HTH.