I was only speaking to the basics of restoration. Most people think that they need to resort to a chemical, when there are other methods out there that have zero chance of harming the coin. It is also best to try these methods first to see how the coin and contaminant react.
tapwater is usually chlorinated to some extent. it's also usually mineralized. I'd agree start with distilled water, it's a good solvent and won't hurt anything. then you could try the olive oil. just a light coating, you could use a soft animal hair brush for that, as far as trying to care for and restore old neglected copper. if these things fail to correct the issue, you could try something harsher, I hear denatured alcohol can remove light tarnish just by wiping with a cotton ball. it will kill off biological matter that might be on the coin surface also however, I don't know if that will change the color of the copper, and whatever you do, you should always neutralize afterwards with distilled water as a rinse in my opinion. distilled water soaks will dissolve solids and remove encrustations without damaging the surfaces, then a rinse and pat dry, and repeat. the olive oil thing is more about maintaining the patina, and protecting the surface with a light coating from contact with ant contaminants in the air, it's more of a preventative, than a restorer. temp can also be a factor for growth as well as humidity, light and darkness. if the coin was buried, there can be all kinds of spores on the surface and bacteria/molds/ect. just lurking for the right conditions. below freezing or over 145F can inactivate most everything biological. drying it out really good in the sun can kill off molds though exposure to ultraviolet light, but it wouldn't stop it from returning if conditions were right if even a bit is remaining on the surface. the coin capsule will also have spores in it if the biological matter that has grown on it inside the capsule. so don't bother using that again unless you sterilize it also. likely it was put away before it was sterilized and dried out and was still a bit damp, shortly after being pulled from the ground. if sterilizing it, likely the acetone or alcohol would do the job ad kill off anything biological on the coin surface, but temp and time, either hot or cold can work pretty well, and not even extremes, just more time, so could a UV sterilizer for that matter. anyways, it's an interesting problem, take it slow, take it easy, try to figure out exactly what you are dealing with and the best way to eradicate it doing the least amount of damage to the coin. distilled water for instance won't kill off biological material, however it should remove mineralizations and encrustations that might hold the biological material to the coin and remove it along with it.
I don't use anything in my coins. I figure they are all old enough to be abused by everybody else. And if I sell something I don't b like every one of my ads to say p0ssibly cleaned but if I suspect It ill put it. Usually the reason Ib like BU coins. There isn't much reason to abuse those. In picked up some beauties recently Im trying to get rid of my collection but if Iv sell one I buy another one Collectors craziness.
So I didn't do anything to this coin other than brave opening the case and I wanted to follow up with everyone that posted here and show you this coin. I was actually pretty amazed and pleased of .
Thought you would find this very interesting since through the case it looked extremely fuzzy with growth.