Dirty cotton... I have a little sponge, it is yellow and tiny, but I can still see lots of little pores in it, it also has a little tube coming out the middle so it looks like a little nipple on the rock, all sponges have a tube of some sort, whether they grow in a tube shape or it sticks out, because the draw water through the main body tissue, through the pores, and feed on things in the water, then they pass the waste water back out through the big opening tube, called the excurrent siphon.
Crabs and shrimp may eat small sponges, after all they are inclined to eat anything they can, as for fish I'm not sure many species do,
sponges seem an easy target as they are fleshy and can't move, so they are usually toxic in some way, (fun fact lol)
If we know what you have in your tank it might cut down the research we have to do to find out what might have eaten it!
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