The very first line of the skill description reads (roughly translated, I don't own an english book)
You use the spot skill to find characters or creatures that are trying to hide.
So yes, if the moon was a living being actively trying to hide, you would get a huge negative modifier. As the moon is not actively hiding, because it's just a mass of rock, you don't have to roll spot at all.
That said, somebody the size of a moon would probably grant a huge positive modifier, even if it were actively hiding.
Hide modifiers are:
Fine Diminutive Tiny Small Medium Large Huge Gargantuan Colossal
+16 +12 +8 +4 +0 -4 -8 -12 -16
The scale ends there, with colossal defined as 64 ft height or more. Given that the moon has the height of twice it's radius of 1737.10 km and a kilometer is 3280.84 feet, that would mean the moon would be 178098.34 categories above colossal. So if it actually did come to live and decide to actively hide in the clouds, he'd indeed stand a good chance not being spotted from earth.