I took a sample of it and i know it sounds gross but i tasted it. It is definitly salt. Do you live where they use salt on the roads?
My concern is down the road in a couple of years what the result of the salt will have on the door itself. I am calling the Subaru Customer service the beginning of the week and see what else i can do about it. The SM at the dealership told me it was the first time sombody complained about it, but the Subaru CS told me the SM said there were several complaints.