I noticed these spiky worm on a live rock i bought from someone shutting down their tank. I never noticed these before but i turned up the light and noticed them.
Yes. Bristle worms. They are good at cleaning extra food and waste that make their way to the bottom of the tank of inside rock crevices.
They are harmless creatures but, those spines are defensive and sort of like fiberglass strands. They look soft but it’s not. They are sharp and flexible and will stick in your skin like a splinter. Just be careful around them and use forceps/tweezers if you ever need to handle one.
They generally hide in the daytime and you won’t even notice them. Might come out during a tank feeding but they will retract and hide shortly after. They will come out in the dark and clean while you sleep