Northern lite blind

Due to a house fire we only got to use our 2019 10-2 dry bath once bringing it home. I winterized it and kept a large house size dehumidifier on 24 hrs a day. the stove and sink have yet to be used. The blind behind the stove developed a silver dollar sized white spot. I brushed it with a soft brush it is not mildew or any type of mold. Northern Lite had no idea what caused this strange reaction either. Northern Lite provided better service than I ever thought possible, U P S delivered a new complete blind. have not took it out of the box yet has anyone else ever replaced the blind in a newer northern lite? I sure would like to know what caused the problem I will try to insert a picture of the old blind thanks NORTHERN LITE.