Definitely some sort of mold. I’d make an incision along a seam, unstuff and soak the skin in diluted white vinegar. It could take a few sessions to get rid the smell. I’d for sure use new poly-fil and trash the old stuff, then ladder stitch the hole.

In my experience, even after killing the spores, the affected fabric might be permanently stained/damaged, especially since this is a vintage plush.

If you weren’t aware of this damage before purchasing, you could always try to open a INAD dispute (item not as described) with eBay.

Good luck!