It's fungus that grows on damp wood. Since the wood is damp enough that it's growing fungus, and you have newspaper as a backup bedding option, you can either throw the bark away or cook it in your oven for about two hours to dry the wood out and kill the fungus. Since I'm not inclined to throw away money, I'd bake it and then it should be fine to use as bedding. It's up to you, though.
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