Before desaturating, use the Magic Wand tool to select the sky region (less the moon). Save the selection and then desaturate the entire image. Next, load the saved selection and apply the Filter/Noise/Median command. Since there isn't any detail in the sky, averaging adjacent pixels won't do any harm. If you had wispy clouds in the frame, however, it would be a little more complicated.
As far as silvery goes, I think that's primarily the domain of film emulsions. There probably ARE printing technologies that can mimic the effect, but we amateurs may be SOL.
-Anthony