Spirorbid worms.
They're a filter feeding species of worm that creates a spiral shaped shell permanently attached to a solid surface. They live in that shell pretty much their entire lives. They are beneficial. They eat free floating debris that would otherwise decay. Its also a good sign your calcium levels are appropriate and that you may be able to add LPS or LPS to your reef! if they get on your glass where you don't want them you can just scrub them off. Otherwise, they won't hurt a thing.