Is this frost or frozen bacteria in clouds?
If it disappeared after you descended, it was frost specks on the inside surface of the structural* outer panel, forming on dirt particles on the plexi, from the small amount of humidity in the cabin air that migrates through the little vent hole in the inner panel that is part of the interior fascia.

It's not a problem on cockpit windows because they are heated.

*I say structural, because the outer pane of quarter inch thick acrylic has about the same tensile strength as the adjacent much thinner aluminum skin and is part of the pressure hull.