Since the Crabpack has reappeared, I noticed this skin and, after searching, discovered that it was originally part of a game mechanic. Around 2017, a regular backpack would decay after being left on the ground for a while and transform into this Moldy Backpack. However, this mechanic was later removed, though the backpack skin itself seems to have been retained.
It appears that Klei has not provided any way to obtain this skin, making it likely the only skin that no one has ever acquired. I saw its appearance in an old video, and it looks absolutely adorable. I wonder if Klei has considered making this skin obtainable. I really hope there will be a way to acquire this fun and cute skin.
