We're not certain what's causing this symptom, but it resembles edema, which occurs when plants stay too wet. (The bumps do not appear to be caused by a pest or disease.) Consistently-moist potting mix, especially if also combined with high humidity or low light levels (which suppresses photosynthesis, which uses water), can predispose plants to developing edema. Although Christmas Cactus is a type of jungle cactus and not an arid-climate cactus, they still can get too wet if watered too often, as they should dry out somewhat between waterings. If the plant sits on a saucer or inside a cache pot, empty it promptly after watering the plant so that the collected drained water doesn't seep back into the pot and keep roots too wet. If the pot doesn't have drain holes, consider transferring the plant into a pot that does. Clemson Extension has some growing tips for these types of cacti, if useful (see the linked page). Growing them in bright light, either from a sunny window or under grow lights, will help to avoid edema by supporting good levels of photosynthesis and growth.

Miri

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