In the photograph, it appears to be oxidation, possibly from contact with a surface where it was stored......the case interior or a holster or gun rug?
Could be dirt or debris though. Look at it with a magnifier to try to determine if the specks are actually on the surface or a part of the plating.
Many here on the forum use metal polishing products like "Flitz" for removing tarnish and surface dirt from plating. Remember to follow the label directions carefully and use caution. Metal polishes usually contain a mild abraisive, and ammonia in some form. If the oxidation has penetrated the surface of the plating itself, you will not be able to remove all of the damage. The electro-plating coverage is very thin.....only .001" - .002".
If the plating is damaged by oxidation, this would affect the value.