Some bristleworms turn into nothing more than swimming sacks devoted to reproduction (digestive systems disappear, and so on). These 'epitokes' then swarm to the surface at night, and disintegrate to accomplish their reproductive mission.
What you have seen is not quite an epitoke: a portion of functional worm remains, while the portion devoted to the storage of gametes has disintegrated (releasing its cargo). Some species of worm just do things this way.
Blunt skippy: your bristleworms have attempted to reproduce.
You did the right thing: when confronted by the unknown, play safe (so long as it does not disrupt the tank overmuch. But knowing it merely to be bristleworm repro, you should leave it alone next time
horge