There was a glob of something on the face and when removed it had eaten the cladding. I guess the first one stays in the collection!
Interesting, I could see how that action could turn out real bad for any coin. It would definitely clean the area though.
Welcome to CT @Sharkbait46737. Nice to find "special" coins in circulation, and as mentioned, your's also has the "victory" mark, which adds to its keepability. You might also try using 100% acetone (not fingernail polish remover) on that stain and see if it removes all of it. Let us know your results in a separate post.