I think it's a..........

[grouper]

I think I saw a have some kind of worms that only come out tonight. It was yellow with spikes like things around it. I'm unable to take a picture, it's impossible. When I put the light on it, it ran back into the rock hole. it was no bigger than #2 pencil lead. It was tucked away behind one of my nems.
Could that be a bristle worm? Are they harmful? How do I catch it? What kind of critters eat those things.

 

[mvm]

Most likely a bristle worm and they are harmless. Beneficial, actually.

 

[george]

If they reach plague populations (which probably means you're overfeeding since they're detritus feeders), you can get a coral banded shrimp or, less preferably, an arrow crab to eat them.

However unless you get an explosion, I'd leave them in there. As mentioned, they are very beneficial for cleanup and substrate aeration.

 

[linda lee]

Beneficial to your tank, but can give you a pretty good sting and some people have nasty allergic reactions. 6Line wrasse is good for population control if you start to have too many.