Not rust. Did you use Bug Spray, Sun Block or something else while shooting? Looks to be over spray from an aerosol can or splatter from another chemical. If gun oil or a safe solvent for the Walther finish won't remove it - I'd try and live with it otherwise it might have to be refinished.

I once got a Carbon steel pocket knife all stained up by just cutting an apple. I eventually got it all off using Flitz but I am not recommending Flitz on your gun as I do not know how it would react with the finish.