Oxidation. I assume there is air in the room^^ but you can just clean it with your T-Shirt. Maybe not use the most expensive shirt as this could end up on the shirt.One question to the owners here: The balanced cable is new, never, used, kept in a cool room with no drastic temperature changes.
Why does the cable look like this after a while?
But there is basically nothing you can do against it. Maybe seal it