WHAT IS THIS THING?!?!?!

[thatguysulli]

Before i took it out of the sandbed, it was long and stringy spreading over the rocks. I'm sure it's a parrisite but is it harmful to my coral?

 

[JGT]

Spaghetti worm. Harmless. Good CUC.

 

[ISpeakForTheSeas]

I always struggle to tell which from pics, but, either way, it's harmless:

Looks like either a hair worm (Cirratulidae) or a spaghetti worm (Terebellide) to me - if the tentacles all come out from a single point, it’s a spaghetti worm; if they come out from basically all over the body it’s a hair worm.