Sponge of some type?

[jgvergo]

Help ID'ing the white semicircle/spiral things please. For scale, the shell is probably 1", so they are really tiny.

 

[mamedina]

Look like little feather dusters...beneficial filter feeders

 

[Mikedawg]

Look like spirobid worms; harmless filter feeders if so. Sometimes you'll get a lot of them on glass and if you don't want them just use an old credit card to scrape them off.

Good guide for hitchhikers: https://www.lionfishlair.com/hitchhikers-guide/spirorbis/

 

[Cnidoblast]

spirobid worms - harmless but annoying - if there is nobody home in the shell i would just chuck it