I just found this forum after a search on the green slime. I've seen this stuff twice recently, but it was coming from electrical plugs on two devices: a very old movie projector and a fairly new coffee maker. Both were stored in my basement for about 5 years, and both had the same substance oozing from the plug ends of the power cords.
This is thick, transluscent blue/green goo, and I couldn't find anything that would desolve it (mineral spirits, amonia and soap & water, etc). It looks almost like melted hard candy.
I know what oxidized copper looks like, even mixed with water, and this certainly isn't the same. I was wondering if maybe radon or oil-burning byproducts were reacting chemically with the insulation and/or wire. I still have the coffee maker, and I might take it to a local college chemistry lab to see if they have any thoughts. I'll post the results if/when I get them.