Looks like gunk amongst the devices. You'll have to crack it out. Soak in acetone and use a Q-tip to gently remove. Personally, I'd leave it be. I'm guessing it's in a problem free slab?
I just blew the pics up larger. Yeah, slight verdigris issues. verdi-care spot treatments should work.
Yes, it will progress in the slab but don't crack it out of the slab. Contact NGC and tell them you have a coin in their slab that has developed verdigris. If they are like PCGS they will buy your coin back at full market value.
I own ZERO slabbed coins, so someone who is more familiar can maybe correct me if this is "run awaaaaaaay!! " advice. But here is what I would do:
Write to NGC. Tell them you have a coin in one of their slabs that has developed verdigris and that you would like to conserve the coin, and inquire if they will waive the slab fee to reslab after the verdigris is removed. The worst they can do is say no.
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