I don't think that is limited to just Starline brass, I have seen similar discoloring from time to time on just about any brand of brass you can think of, even once on brand new Federal ammunition.
I'm not sure what it is, but I have taken it off with a scotch-brite pad before polishing. I store polished brass in ziplock bags and put in a desiccant pack.
Yesterday 05:49 PM
This is a problem with corrosion. The corrosion could come from many sources. Sweat on your hands is corrosive and the most common way corrosion gets on brass.
As far as Starline, IMHO and particularily in 45 Colt, Starline is superior to all domestic brands. Key word: Domestic