What causes this moving patch of clouds in my vision?

Vitreous floaters? I suspect you are describing vitreous floaters - small bits of debris in the gel that fills the back of the eye. These are common, and are often described as looking like a gnat or hair that moves when your eye moves; more prominent looking at a blank background like the sky. New floaters, esp if ass'd w/ decreased vision and/or light flashes should prompt evaluation for a retina tear.