That’s the onset of the dreaded white worm corrosion! All too common on diamond cut / machine polished alloy wheels - usually when the wheels have been on the car 2-3 years as yours have. It happened on my mk6 Golf at 30 months, in spite of my wheels being meticulously maintained - always well protected with sealant and wax, and any stone chips to the lacquered surface being touched in with clear lacquer as soon as they happened.
It looks as if it’s started along the edge of the machine polished spokes and is probably due to poor (or thin) lacquer coverage on the edge of the spokes during manufacture, which has allowed water ingress to ‘creep’ under the lacquered surface.
Lacquer doesn’t adhere very well on diamond cut / machine polished metal surfaces, so this type of milky white ‘spidering’ across the surface is all too common. Unless you replace the wheel, getting rid of it requires complete refurbishment of the wheel (cost circa £90+ VAT per wheel for a good quality refurb) which involves re-cutting the face of the wheel and removing a layer of metal. Because you’re removing metal with each refurb, diamond cut wheels can usually only be refurbished a maximum of two times; beyond that, the structural integrity of the wheel can be compromised. Additionally, even if diamond cut wheels are refurbished, theyre likely to suffer from the same corrosion again in another couple of years time.
Diamond cut alloys have great showroom appeal on new cars, but they have poor durability and just aren’t designed to withstand the type of weather conditions we get in the UK. Thats why I changed my wheels to a set of powder coated alloys - much more durable, and no risk of white worm corrosion.
Given the choice, I would never have diamond cut / machine polished wheels; I dislike them with a passion! Just my opinion though...........