i was thinking for filter feeding. but not about waste removal. it was really surprising to me because the tank has been set up for a while with no signs of these things., then 1, then 10 or however many they are. same with those round, white, feather duster, things that take over the glass. they exploded, stopped, and now they are slowly coming back. its crazy what happens with life in such a small space.
Everything that eats eliminates waste and is the reason there are groups of aquarists that do not keep clean-up crews. Having gone through multiple attempts at keeping a saltwater aquarium using the pre-liverock, serile method vs the multiple long-term successes I have had with encouraging nature's critters to help biodegrade what I don't or can't remove, I'll keep my clean-up crews.
I have several tanks over 4 years old and I rarely put aged or live substrate (like LR, I do actually change out the bottom substrate every few years but with dead sand) in them but I still see new things growing. What I don't get is much propagation of the mobile critters (like urchins) and hope to study more on husbandry ... eventually.
For a while we had some nice bright yellow sponge growing on our back glass in the tank the cuttles are in. There is a very large transparent white sponge in Kalypso's tank. It looks very much like octopus skin with a big syphon sticking out of it.