Certified GKH Gardening Expert
This is powdery mildew. This is most commonly caused by overwatering, high humidity, or lack of air circulation. You can correct this by spraying with neem oil. You can also add just a little wettable sulfur to the soil to keep it away, and prevent it in the future.
This article will refresh you on the care of the christmas cactus: https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/advice-for-christmas-cactus-care.htm