Water the soil well before cleaning the wall.If you have a hose end sprayer that has never been used for weed
killer use that to spray your solution. Add a cup of your cleaner and fill the remainder of the sprayer with water. Add an old golf ball to keep the solution mixed. Buy shower curtains at the dollar store and cover your plants that face the wall. Remove the shower curtains ASAP. Wash down the plants well with a jet of water after the wall has its final rinse.
What caused the rust stain? The sprinkler or your water source? Or did the rust come from the plants? If the rust is in your water then you wouldn't want to use the hose end sprayer.