As much as you touch up the holster wear it will just keep happening. If you want your Glock to stay looking new, buy another and don't holster it. Holster wear is just one of those things you need to accept, there's no way around it. What I will add though is from your picture that looks to me like some sort of solvent was left on it, or even sweat.
My firearm looks like crap.
Any suggestions?
If the gun fit in the holster is too loose, you may get reduced holster wear, but - you get no retention, which is not good.