Israel Mint Set Goo

The holders are clearly worse than useless at protecting the coins - I'd strip the plastic off and throw the holders away.

The corrosive agent appears to be coming from the cardboard inside the plastic, rather than the plastic itself - probably acid, or bleach, or moisture. There may be a component of "PVC damage" and you could try using acetone to remove it, but I'd say the metal has started to actually corrode underneath.

I've seen identical-looking damage on some of my late-date French cupro-nickel coins. The green stuff is difficult to remove, and where it does come off there is corrosion damage to the surface. I hope this is not the case with your coins, but it's a possibility.