definitely a bristle worm. Alot of people think they're a menace to have, but there seems to be a growing trend of people who are happy to have them. I have a monster of one in my orca nano, like 4-5 inches. I only saw him once but he hasnt eaten any of my fish or bothered my corals so i'll just leave it for now
Bristle worms can soon go out of control in a nano tank, but if numbers are kept in check can be useful. Try filling up a a small piece of a womens stocking/tights with a piece of bait (cockle, mussle, shrimp) with a small piece of rock to weigh it down. Tie up the ends to form a bundle and go bristle worm catching. Leave the bundle in tank at night for about 20min, then pull up the string to see how many you caught. The bristle worms bristles will get caught on the tights so will be easily removed. If you catch over 10 in an hr you may have a problem. All you will need to do is repeat the same method several times a week till the numbers you catch are very low. Remember not to remove these worms with your fingers as they will sting you and may cause a rash...many ppl picking up live rock filled with bristle worms will know the pain it can cause.
I like bristle worms and have never had a problem with them. They are shy and tend to be seen early in the morning the most, as they are detritivores most people think they do alot of harm but its my beleif that when things are going badly in a tank and things are dying you tend to see them taking advantage and cleaning up. When people blame the worms they dont always look to see if there is another problem and other stuff can die.
a few are good... lots means a problem, usually overfeeding from what i've read. I dont even know if my big long one is still alive or not. I havent seen it since the day i first spotted it, and now I have a yellow wrasse thats supposed to eat them so it may be long gone.