If its a polished part - like the way the stainless Springfield loaded models are - you can get a red scotchbright pad. You rub it back and forth, horizontally EVERY time - you basically rescratch a new grain into metal. You need to do both sides to make it look right.
With a bead blasted finish on stainless, there is no fix other than getting it bead blasted again...
As for future knowledge to keep it from happening again:
For "idiot scratch" protection (what that scratch by the slide stop is called), I use this:
I previously had an Ed Brown 1911, and the slide stop went in very easily. But on my Custom Shop Springfield, that plunger makes the slide stop VERY hard to get in. EVERY Springfield (various model 1911s) I have previously owned was the same way. That slide stop won’t go in easily because of the plunger…
So, because I DON’T wanna screw up even ONE single time on a $3k gun - I now use that protection device (I put the link above).
I lay it (that small device) on the frame, and then use a small screwdriver to push in the plunger while I seat the slide stop. Then, pull out that plastic piece afterwards that protected the frame.
I used to use scotch tape and some other tricks. But now, I just use this. As it is more reliable and works perfectly every time.