What is this?

Looks like the arm of a starfish.

That would be a bristle worm.

Would that be a good thing or a bad thing? I think I saw two of them in this peace of live rock I just bought!

depends. some people like them in their tank. don't touch it! great shot of it! they creep me out!

Why do you say dont touch it! lol Not to say I would because they are kind of creepy!

Back in the 80's they were called "Thunb crushers"...if you let it brush you, you will understand why....

May be a brittle star arm too....I would prefer a bristle worm over a brittle star in my tank....the stars can get pretty predatory when they grow....

I am glad someone else thinks its a brittle starfish arm. I have lots in my tank and they wave their arms in the current looking for food.

definitely a bristle worm

Thats a BW. If you do ever touch one soak the area that your frist though was to cut off in vinegar. It dissolves the spines and helps with the pain.

they're geneerally harmless scavengers, some are venemous as stated ^

I wish I had knows to soak my hand in vinegar. I was moving rocks and it hurt like a b!tch.

Definitely not a star arm but a worm.

I would keep it as they scavenge detritus and can get actually quite cool colours. I have a 7incher that's really cool shiny cobalt blue.

+1 bristle worm pretty harmless, i touched one years ago with gloves on & its bristles still went through the gloves! was silly enough to try n grab it ....bad idea lol.

yup, +1 on the BW Ive touched them before, several times, nothing happened to me... but IT IS A BW!

i have a lot of these... they hurt!

Its a bristle worm i seen them come out of the rock last night there is two of them they are about 6 in long! I dont have any plans of taking them out of the tank... I know that there is a few fish that will eat them...

Don't kill it, they help keep your tank clean.