Can anyone identify these things?

[Kit with Cat]

I found TONS of these things while cleaning out my Rena XP filter..they feel like sacs? I am not sure how to describe it otherwise...anyone know what these things are?
My water parameters are good. Nitrates and nitrites are zero (I have a fuge setup with tons of macroalgae). 90G tank, a blue regal tang (1.5 inches at most), 2 false perculas, 1 bicolor dottyback, 1 diamond goby. Corals, hermit crabs, snails, 2 cleaner shrimps, 1 peppermint shrimp, 1 cowrie, 1 tuxedo urchin, 1 emerald crab, 1 pom pom crab, 1 super small yellow sea cucumber.
Thanks everyone!