This will make you think about

[fkshiu]

I do believe it's a eunicid of some sort.

They can grow a fair bit bigger than the beauty that you've got if you leave him be:

http://www.oregonreef.com/sub_worm.htm

[untamed]

It is Eunicid of some type. Without identifying the species, it is impossible to say how large it could get. I think they are incredibly common hitchikers. In fact, I've never had rock that did NOT have a few.

Mostly they are opportunistic feeders that will eat anything they can grab. I kept a similar looking one for many years that was vegetarian. He was called "Max".