That is Azalea leaf and flower gall (Exobasidium vaccinii), a fungus. The white growth you see are the galls, which are made up of fungus-infected plant tissue. You should remove and destroy the galls before they turn white. Avoid overhead watering and use drip or other low trajectory watering method that doesn’t wet the foliage. Make sure to prune to keep the plant open and improve air circulation, and make sure the plant isn’t too crowded by neighboring plants. For more information, refer to: https://pnwhandbooks.org/plantdisease/host-disease/azalea-rhododendron-spp-leaf-flower-gall