I have never seen a bristleworm that big before.
The typical ones that hitch hike on rocks and corals are small and are mostly opportunistic feeders (they only eat dead animals or those already dying).
There are bigger ones that are known to be active predators but very few people had the misfortune of acquiring one into their tanks.
Saltwater factoid: Marine fishes sleep at night, freshwater fish rarely do.
This allows opportunistic predators to catch a fish completely unaware and without much effort.
This is why if there is a predator in your saltwater aquarium, most disappearances occur at night.