Answer: Your photo wasn't close enough for me to be positive, but I think it is corky scab. Many species of cactus develop irregular spots or patches with a corky brown appearance. In most cases, these spots are superficial and don't penetrate beyond the epidermis. However, shoots can sometimes be covered with many scabs and appear unsightly. When cracks develop in the corky tissues, the epidermis can break open and curl.
This problem is usually not serious but can be prevented by increasing light exposure and relative humidity levels.
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