What are these?

[Joshua2598]

I have these little almost shell looking things on my back panel that separates the sump from the main display on the fluval evo xxii. Some of them have an almost fuzzy appearance

 

[Crabs McJones]

Spirorbid worms. Filter feeders. Generally harmless, but can populate quickly especially in the back chamber.

 

[Joshua2598]

Thank you