Depends what it is.
If it's Verdi Gris (green corrosion) this is brass oxides forming, this can be just discolouration, but if there are pits and a white substance too, then it goes in the scrap bin.
If it's black or brown spots, that is contaminant corrosion, like what happens if brass is left in a damp leather pouch or belt. This is harder to remove but poses no problem with integrity of the brass.
This is caused by the tanning salts/chemicals in the leather.
For stubborn stains, 0000 steel wool works fast to remove them.
Water marks are also easy to remove with steel wool.
Cheers.