That sucks! Where do you live as I'm sure the environment in which the car stays in can easily cause this? I'm not sure how possible replacing the mirror would be and if there would be more to it than that, but asking your dealer wouldn't hurt. They also make a lot of plastic restorer products that I'm sure could help improve your problem. Check out autogeek.net for some products you could use.
Black WOW Exterior Trim Restorer, black trim dressing, restore black rubber trim, Mini trim
OP is from Singapore, can't learn this by reading.
Could be dead wrong, but under the impression these plastic pieces are molded from black plastic. If this is true, should be able to polish them and get back to black. Because this should be black underneath.
If painted, that would be a problem. Have no idea what that white stuff is, but could make terrible suggestions. Used that Armour stuff on all the plastic inside of my Cruze, many different brands of that stuff all claiming to be the best. Someone is lying. Anyway, not a problem, have to do this at least once per year. Also 50 millions other things you have to do once per year. Now its taxes.
My own practice was to polish the car twice a year with a good polishing service and wash the car with just plain water. In Singapore, there is no 4 season, just humid. We get a lot of sun, rain and temperature is between 27 to 32 degree Celsius here. Tried using armour oil for rubber or plastic, cleaners similar like Mr Clean also don't work.
I've sent to Service Centre but all they did was re-spray. I've demanded a replacement and they said the issue would come back again. That is why I posted here and see if anyone have contacts with GM side to check it out for me if it is their manufacturer issue or just an excuse from the service centre for not replacing. I've only just bought this for just over a year. I don't seems to have the same issue over my Japanese car from Toyota, Honda or Subaru!!!!
Cruse member from Singapore