StarlingWings is right
. I’m sorry your budgies have come down with Scaly Face mites. Even though Kiwi doesn’t show the infection, as mentioned, it’s highly contagious so chances are they both have it. Skippy’s case is advanced enough that you see the crustiness.
The Spot On Ivermectin treatment is effective. Follow the directions on the package. If you can’t buy Ivermectin with dose instructions for birds at your local pet store, then they sell it online. Good luck