Shoot steel or aluminum shelled cartridges no brass marks
Sorry, I don't think there's anything you can do about that. Playing with the ejector (when pistols function is fine) might be inviting trouble. Have a Glock that does the same thing, and don't worry about it anymore.
On my H&K USP Custom Sport I get the same marks.
I use Milfoam to remove it http://www.milfoam.fi/flash/index.html (Takes less then 2 minutes)
Im waiting for a SA Mil-Spec Parkerized and I guess it will work on that to.
There actually are ways to remove the marks without damaging the existing finish. Get a "copper solvent" cleaner and take a small patch and saturate it. Place that small patch over the ding marks on the slide and leave it on there for a few hours (or overnight). The marks will be gone. You can slightly speed up the process by using heat. I have a bench light over my cleaning bench and place the light about 8" from the slide when I do this.