These moon cactus are designed to die and that is what is happening. . I see them as the entry cactus that suckers the naive. Rarely will these live long enough to bloom. It is a grafted cactus on the green root stalk. This cactus does not make chlorophyl.

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Here is a link that might be useful: Moon cactus

Moon cactus once it looks like this it most likely want live long but u can sometimes save them by grafting to new green stems again but not for a beginner to do u may be able to get one of the babies to grow roots if u put them in vermiculite but please don't hold ur breath for them to live most don't sorry