poss. oxidation of the clearcoat
had something similar on an older car... wound up stripping the clear coat from whole rim & just kept them polished afterwards.
This is exactly what it is. GM OEM Factory polished wheels with clear coat on them are notorious for this. Depending on how long you've had your vehicle you may get a dealership to help you out on replacements under the "corrosion/rust" warranty but it's probably a long shot.
It will only get worse and progress from here, soon you'll start to see it in the corner pocket of the spokes, until you either decide to refinish them or get different wheels.
Though polished aluminum wheels without a clear coat look great; Maintaining bare polished aluminum wheels without a clear coat is very time consuming for a daily driver, something I would never sign up to do. If you like the polished wheel look and want to keep it. Get them stripped re-polished and have a transparent clear powder coat applied to them. It will last much longer, be very shiny, and a lot more durable.
