White stain / discolouring of iPhone 6 Plus

I bought my iPhone 8 months ago and I've had it in my silicon case from day one. Never dropped my phone or scratched it. I noticed the other day these white stains on the back and the side of my iPhone. I tried to wipe some water and soft cloth, it does not come off. Went to the Apple Store and they tried to clean it, does not come off. The guy at the store recommended I make an appointment at the Genius Bar which I will. I'm still under warranty. Is something like this covered?

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