That looks like an oxidation problem. The center cap and the wheel might be made of different metals. When water touches them the metals react with electricity, and the react usually causes surface defects on the metal. Try to change to a different wheel cap.
it looks like the clear coat was damaged during tire change. Then the metal started to oxidize. I believe the center caps are made of plastic so the two different metal theory is out. This is just a guess.
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