My space gray iPhone 6 is discolored!

So I purchased my iphone 6 space gray beginning of February. And ever since I've had a protective 3 layered case I ordered from amazon for about $10. So I always take care of my electronics, and I am very careful with them. It's always usually in my hands, on my desk or in my bag & pocket with no exposure. But about 2 weeks ago, i took my case off and seen residue around the openings of where the case usually is exposed (the camera and apple logo) as you see in the pictures below. I tried to wipe it down and it not going anywhere. Nothing I do is removing it. I even tried using an eraser ( a known trick for iphone discoloration) and it didn't work. I honestly have no idea what it is or where it came from. I would've remembered if any liquid spilled over it but I don't. I showed my sister and she told me it was the case eating away at the metal. I think this is the only possible explanation.

I'm not sure if there is anyway to reverse it. Is this familiar to you? What do you recommend I do? Please all suggestions are welcome btw the phone works fine, the stain just bothers me a lot.

Thanks (: