Looks like a mild strain of fowl pox to me, you're fortunate.
To help prevent mosquitos; eliminate standing water where possible. You can add sand to the pen or run to soak up water, it wont wash away neither. Put fine mesh screen on vents to your coop(s.) You can hang screen over the entrance to the coop and cut verticle strips through the screen allowing your chickens to walk freely in and out of the coop to lay eggs, go to roost at night etc...