Answer: It's hard to tell without seeing the plant whether it has scale insects or fungus growing on it. If it's in a dry area, I'd suspect scales. If there's a lot of humidity around the plant, it could be a fungal growth of some kind.
One other possibility might be hard water residue if you've been splashing water on the plant as you apply water to the soil.
Scales are sucking insects and unless removed will effectively drink the moisture out of the tissues of your plant. They can be picked off with fingernails or a pair of tweezers. The pests have a hard shell which protects them from insecticides, so picking them off might be the best control measure you can take.
Fungus is a more serious problem. Your plant can be salvaged if you're willing to do some surgery and cut out the affected portions. If you do this, be sure to correct the overly humid conditions around the plant so it won't happen again.
Good luck with your cactus!
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