I looked a little closer at the packaging, and you can see a little break in the seal at the seam and the white scale making a trail in from there. So it literally could have been anything. Spill, mildew, moisture, who knows. I was under the impression it was sealed, so that made me think it would have had to be something common.
Glad it cleaned up nice! Guess I need to look closer and not assume "Sealed" always means a nice looking coin.