Answer: There are two likely causes of white spots on Christmas cactus; fungus or mealybugs. If the spots are a fungal infection, it's probably from growing under too humid a condition. You can use a fungicide manufactured especially for houseplants to control the problem, then correct the conditions that lead to the infection by increasing air circulation around the plant. If the problem is mealybugs, you can remove these sucking insects by wiping them off with a cotton swab dipped in rubbing alcohol. Frequently check both the under and upper sides of the leaves and remove any new little insects as they appear.
If your plant isn't too terribly affected it should recover.
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