Allow me to be the first to respond with the usual questions:
What is your budget?
How realistic does it need to be?
What other design/style choices are being made that this solution has to work with?
Options range from an actor with a cloud cutout on a stick running across the back of the stage to moving backdrops, projections, to video walls. In my budget range I would be looking at animation wheels, GAM Film/FX, and gobo rotators.