I had the same thing happen with a couple of grills in had in my garage. They both had recently been cooked on. Never have it happen outside. Probably has to do with conditions being perfect for mold to grow. Warm, with no air circulation. Outside, there is always enough wind and air movement to make it inhospitable for mold to grow. I have quite a few grills in the garage and it has only happened to ones I recently cooked on. Ones that had old rusty grates I have not restored or cooked on never get mold on them. They sell machines for marine application that have little fans that circulate the air. They prevent mold inside boats cabins of boats and houseboats.