clearly, some form of
crustacean

*ahem* sorry, I couldn't resist.
They're spirorbid worms... harmless filter feeders, aka free filtration, good for biodiversity. Strong indicator that your water is full of excess nutrients and these guys are picking up the slack.
If they're getting
in the return pipes and blocking flow, they should be pretty easy to scrape free with a stiff-bristled brush or even a screwdriver or butter knife, but otherwise I wouldn't worry about them.