Water pressure does not appear strong enough to power the upper spray arm. I pulled the unit out and the water valve assembly had some black rubber gasket bits in front of the pink screen, and I'm hoping that's the problem. (not sure what the function of the water valve is.)

Or should I check the impeller on the circulation pump before re installing the machine? Is a pump rebuild kit effective or do I need a new one?

If you just noticed that the upper spray arm is not turning, the problem is probably that you have calcium buildup in the spray arm's holes that let out the water. Without the water moving out of the holes the spray arms won't spin properly. If this is the problem, use a pin to poke through the calcium.