Yep, bristleworms. Some people don't like them, but I think they're great.
I have LOTS of them... some well over a foot in length. They usually remain hidden in rocks.
Amazing clean-up crew and a sign of a healthy aquarium. They will NOT kill fish or harm others. They will eat dead fish, so if you see a worm or five disposing of a fish or other detritus, it was not the worms that killed it.
If I drop an algae wafer into the tank, they all come out looking for it. They have an amazing sense of "smell" to locate food.