Green coating on nickel may be nickel oxide (https://en.wikipedia.org/wiki/Nickel(II)_oxide) but in view of the 400C temperature needed to produce it this seems unlikely. More probably the metal parts of spectacles and concertina are nickel plated brass, and the plating has worn through exposing the underlying metal, which soon discolours.
The particles of nickel which have worn off may possibly become embedded in the skin and cause an allergic reaction (my suggestion, not researched).
I have seen green coatings on (eg) plated violin string adjusters, but this usually can be cleaned off easily, suggesting it is a surface coating of body fat (or motor oil if you do your own maintenance - you can't get it all off without a fortnight away from it!) somehow discoloured by a simlar reaction
Chromium, an alternative plating, can certainly cause skin problems; although this metal usually delaminates and comes off in small patches (see old motorcycle brake levers etc).