miniature grunge

I recently acquired some “vintage” miniatures (these
were the minis I wanted, but couldn't afford back in
the early 80's).  Anyway, I'm very happy to have them,
but many have a "corrosion" on them.  I assume that
this needs to be cleaned off before I paint.  Two
questions:

What exactly is this "corrosion"?

And how do I remove it correctly?

Thanks in advance..

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