They get a lot of this in Europe where they put salt on the roads for the ice. I think it is basically moisture that has got under the clearcoat and "worms" its way towards the centre of the rim. Not sure you can repair it except for re-machining and re-coating. For more info...
How many have been sucessful with Monza II white worm warranty claims? - Tyresmoke Forums
This one has pics...
Killing The White Worm....
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