The bristle worm is an annelid worm of the class Polychaeta which are two words that essentially equate to one — Terrifying. They also sometimes get referred to as Eunice worms. That’s what this thing is and it had been living in this guy’s tank for around two years before he finally found it when he began breaking down the tank so it could be moved.
The poster of the video on YouTube, gurutek, says about the creature, “I was breaking down the tank (as I was moving it), hence it looking shoddy. That unice was in there 2 years before I noticed, and only noticed because I had whole coral colonies missing after a single evening. I first saw it after I spent a few nights sat up (after lights went out) for about 3 hours per night looking for the critter who was eating my corals. Even when I knew it was there, I only ever saw it 3 times within the space of a year. It hides in the rocks, and only comes out at night, impossible to catch without taking everything out the tank.”