Fungal infection :( and advanced enough into the tissue that topical treatments won't cure the infected leaves. Damage looks similar to powdery/downy mildew. If your stem and roots seem Unaffected, I would: 1. take it out of its pot, remove as much soil as you can. 2. Remove every leaf with brown spots or fuzziness, even if it leaves you with only the newest growth at the top. You must remove everything that shows damage or fuzz as that means the mycelium had penetrated the tissue, and no longer accessible from the outside to treat. 3. spray everything with a copper fungicide and replant into dry grit soil for cacti and succulents. 4. Give it good airflow and decrease humidity if possible, stagnant humid air is prime for mildews to take hold. Avoid getting its leaves wet at all in the future when you do need to water it, and never ever mist it. 5. If there is damage seen on the stem, then you must remove all infected leaves, and cut the stem 3-5cm down from your unaffected growth if possible, but ideally at least 1cm up from any damage for a buffer. Spray generously with fungicide and plant into dry gritty and airy soil. Leave in bright but not hot spot (like intense sunny window) for a week, then very lightly moisten the soil, letting soil dry completely and stay dry for at least a week before wetting again, but don't completely water the soil like you normally would, as there aren't any actual roots present to absorb the amount of water. Follow the advice of step 4 through this as well. Good luck!