Answer: Cactus thrives on full sunshine and very little water. Yours sounds as though it's gotten too much water, or the soil is poorly draining. Mold grows on soil surfaces that are too wet, and where there is dead or dying organic matter (perhaps the roots of the cactus?). I wouldn't replant at this point, but would hold off on the water until the soil is really dry. Then water thoroughly and wait for the soil to dry out between waterings. Insecticidal soap will not help bring your cactus back to health - it's not recommended for use on cacti, and it's designed to kill soft bodied insects, which are unlikely to be found on your plant. You'll have to play a waiting game with this problem. Your cactus may snap out of it, or it may collapse. Wish there were something more positive to offer.
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