Magnesium alloys are probably the WORST materials that consumer products of any type and any grade can be made of.
Magnesium alloys are not all that strong, especially compared to some of the wrought aluminum alloys. Magnesium is very soft, very prone to all types of corrosion, probably the MOST corrosion prone material you can find, bends easily just before it breaks, as most have said it scratches easily. Most of the stains you're talking about are various forms of corrosion, which, fortunately, have not penetrated deeply.
With a mag alloy case, don't get within 20 miles of the coast or it will fall apart. Don't eat salty foods and then touch it. don't spill acids or bases or water on it.
If it's a Mag Alloy, run from it as fast as you can. Get Aluminum. Far stronger, more corrosion resistant, less likely to scratch, and generally, a far superior material.
They've almost completely stopped using mag alloys in aerospace. The stuff sucks.
I speak from 25 years as Metallurgical Lab Manager in Aeospace.