It tarnished, or went green
On this page10
Green film on copper
The classic PVC failure: plasticiser and degradation products from a soft flexible holder, migrating onto the coin. It takes years, it is unmistakable, and it is a conservation job to remove.
Tarnish on silver
Sulphur, usually from paper, board, rubber bands or an adhesive sharing the space. Humidity accelerates it and is not the cause, so a desiccant alone does not fix it.
General corrosion on anything
Moisture plus pollutants. The guidance on metals is explicit that both matter, which is why the answer is a dry sealed microclimate rather than dryness alone.
Acid etching from a deteriorating plastic
Deteriorating plastics emit acidic vapours. The metal coupon test exists precisely because metal is the detector, which means your collection is detecting it whether or not you set up a test.
It almost always appears years later
Which is why this is worth getting right at the moment of storage rather than after. Nothing on this page is visible in the first month.
The diagnostic question
What else is in the jar. Not what the jar is made of, which on this shelf is almost always fine, but what went in with the objects: holders, paper, labels, bands, and the liner in the cap.
And the fix is the same each time
Open it, separate by material, discard the doubtful enclosures, dry the air, and reseal. The jar is rarely the problem and is usually the solution.
Do not try to reverse it yourself
Removing corrosion or tarnish from a collectable is conservation work, and the usual amateur methods take surface with them. A coin that has gone green is worth less than it was and worth far less once it has been scrubbed.
Isolate the affected jar
If one jar has a problem, whatever caused it may be in the others sealed at the same time with the same holders. Check the batch rather than the one that showed symptoms first.