It's called Patina. Brass beds do it all the time. I suspect your ammo storage facility has less than perfect moisture conditions. As long as it hasnt changed the size of the casing it should be fine. Since brass oxidizes more in oxygen, maybe sealed ammo boxes would slow it down.
Unfortunately it seems to be my climate. It has been a little rainy and humid here the last couple of days (several actually). I don't have my CO permit yet, so it often gets locked in the car (physically locked away inside a locked car)
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