Odd corrosion/damage on belt buckle - best way to fix?

[RelicHunter]

I just picked up this rather rare Imperial Hessian belt buckle with some odd corrosion damage of some sort on the front. It's difficult to tell if its eaten into the brass, or if its a weird surface stain that's reacted with the brass to form this damage.

Any ideas on the best way to remove it and arrest any possible continued corrosion? I'd rather remove patina on this spot so at least the colors are uniform and this ugly thing is removed. But since its an otherwise rare and valuable buckle, I don't want to make anything worse.

[Strugu]

Spray it with wd 40 and try to remove it with a soft material after some period of time. It will stop the corossion.