iPhone Xs Max space grey discoloration

Did the phone come in contact with high temperatures? Because the only way stainless steel can turn into that kind of colour would be high flames or high temperature or water stain. But my best advice to clean is to use alcohol and a cloth or just simply use alcohol wipes.

I can’t see any obvious staining in your photo. The reflections don’t help viewing, but apart from a few scratches around the charging port it looks like normal wear and tear.