surface deposit
I agree with DAA. I don't think it looks like corrosion. Looks more like a surface deposit. Most organics will include impurities that can coalesce and plate out over time - especially if they had something added like powder residue. If a long soak with a solvent like Hoppes #9 hasn't softened it, then I'd probably go the JB route, but I suspect that the underlying metal is fine.
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