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That is the reflective coating on a mirror that flaked off. This typically happens with much older cars or with defective mirrors. I'm surprised to see this after a couple of years. My guess would be that an inferior mirror mfg. process happened, causing the coating to wear off prematurely.

I would try to claim it if I had a warranty. Ask whether the mirror will be covered; Its harmless. If its not covered then replace the rear view mirror.