Strange green material on the obverse...

[Kirchgoens]

Another reason I think that there may be some sort of polishing compound involved is that that old VL-GPW medal also had the green gunk on the suspension and ribbon. It was very greasy. An odd but effective solution: I boiled it. It removed most of it as the gunk went liquid and formed a layer of oil in the pan. Once the pan cooled, the gunk settled along the edges, making it tough to get off the pan!