Bronze Medallion: Green Spot/Bronze Disease Cleaning Suggestions

Natural toning often caused by the cardboard. Nothing that will damage the coin like verdigris would. Acetone generally won't removing toning, nor would soap and water. The toning may darken and expand a little, but over the course of years if not decades. Not a big deal to collectors usually.

There are dips available that can remove toning. But with those you risk doing more harm than good. I'd leave it as is.

A plastic holder that old is probably the type of vinyl that can cause green slime. So make sure it is not in contact with the coin anywhere.

Great community, thank you for your replies. Regarding my collection, I'm a rookie collector and so far I obtained most of the early Manned Flight Awareness medallions (from Apollo 8 to STS-1). The highly sought after flown Robbins medallions are still missing. My collection is slowly, but surely growing though.
As a child I also found a few coins from Habsburg monarchy period. I gave them to a chemist for preservation and have never seen them since...

Hi there!
That's a neat Medal!
I love aircraft!

So I noticed something.
Your pic with the verdigris spot on it, look at the listing photo that doesn't show that green spot.
On the sellers picture, there is what appears to be a small die gouge just southeast of the N of Grumman.
That die gouge is Not on the picture you posted.
Your pic shows what looks like MD off the front of that Northrop-Grumman AWACS or E2 Hawkeye just to the east.
The sellers photo doesn't show that MD off the nose.
Even the after cleaning photo doesn't show that die gouge.

I don't think the medal in the listing is the medal you received.
That's what I think.

EE