Just in case I didn't make my meaning clear before. I don't think there ever was any reaction of any kind with the silver. I'm convinced that what you've got there is nothing more than a coating of "something" that is sitting on top of the silver. An apt comparison would be paint on metal. The paint causes no reaction in the metal - it merely coats it. And no I'm not suggesting it actually is paint, if it was paint it would have been dissolved by now. But at some point, whatever that stuff is, it was in liquid form, somehow got onto the coin before it was packaged and dried there. I suspect that if you could remove it, you'd find the metal underneath little changed.