What are these worm-looking things?

[Eric19]

Any thoughts what these tiny white worm-looking things are? I came back from vacation and they’re all over my sump walls… They’re a few millimeters in length up to maybe .75 cm.1

 

[Crabs McJones]

fan worms, very common in sumps

 

[BristleWormHater]

These are feather dusters/fan worms, more specifically serpulid tube worms since they use calcium carbonate to construct a tube. They are harmless/beneficial filter feeders.