First question, Did you buy it new? If yes what dealer?
They should always be your first port of call.
I've had mine for just over 18 months, in the first couple of months the plastic alt brake material started gouging the alt bearing, I bought this to the attention of Don Whiteman at Bintel.
He sent me a new set of bearings and the replacement material for the brake.
This had now been changed to teflon. All this was done n/c to me.
So as long as the mirror hasn't been exposed to anything toxic that would cause this problem, the mirror should be exchanged.
As I said contact your dealer first.
