I am starting to get the idea its some kind of factory grease or preservative. I can see a very small amount between the backplate and slide...also, there was a faint amount of this on the slide rails, but that came off easily. Nothing I am doing is making it come off. Maybe shooting a few hundred rounds will help between the heat and gunpowder residue that it will collect.Irizavrima, Welcome to the Forum.
I'll bet a solvent like said or Rubbing Alcohol if you have it might remove it. Like you I'd bet it was something other than "rust". The sight white appears to be glowing.....good.