Probably worms. You may need to visit the vet.
Just for a guess, they're tapeworm egg cases. Get your cat to a vet, just as Ol' Yeddar says. The sooner you do that, the better for your cat. A tapeworm infestation is a death sentence if ignored.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them