Answer: I can't tell without examining the plant, but night blooming cereus is usually disease-free so I'd suspect insects. Try dabbing at the spots with a Q-tip dipped in rubbing alchohol. If the spot is a pest the alcohol will kill it and you can scrape it off. If it doesn't change in color or shape and you can't scrape it off, it is a disease. You can try cutting out the diseased parts and the cactus will callous over the wound.
Best wishes with your plant!
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