its corrosion to the alloy starting under the lacquer.
common with polished and lacquered finishes the clear finish get microscopic holes and lets moisture in starting the process. once its there the only way of getting rid is to have the wheels refurbed.. keep them clean washing weekly with hot soapy water no chemicals afterwards.. my Porsche went the same way after refurb and keeping them clean they were still like new after 5 years when I sold it.