SLABSIDE, A strange thing happens when two pieces of metal rub together. It's called friction and often results in a harmless mark. Sarcasm aside, remove your slide stop then look at the inside surface where it mates with the frame .... betcha there is a casting line to blame. Not to worry ... it won't hurt a thing and is quite normal in all pistols with a slide stop. You can file the inside surface of the slide stop if you want to prevent additional rub marks.
You guy's are killing me, ( smile )
Yes it's rather normal, and a little one at that.
Slide stop goes up and down = rub mark. The parts would have to fit absolutely perfect by accident, or meticulously fitted. The frame has been blasted, so unless you have an air abrader leave it be, it doesn't show.
Or Ink up the back of the slide stop and insert it back in the pistol, gently move it up and down, remove it and relieve the high spot. Be aware this may create a rub spot in another place.
Or just high shine the back of the slide stop in the place it rubs and live with it.