I don't know what is 'wrong' with the cactus, but I can see what's wrong with the soil it's in. Cacti need gritty porous and fast draining, yours seems to be very moisture retaining, peaty and very, very wrong. Plenty of info on here for cactus mixes, have a go with the 'search box facility on here. Maybe that's the cause of your problems, so check it's roots, (and those of the other one) if it has/they have any left! Gill UK

Not sure about this cacti, but amputating the top may save the plant - often new growth starts just below cut area and/or from the stem or even roots. Top seems to be 'goner' and there are bugs on the plant (the webbing is a sign of mealy bugs). Plant needs to be cleaned of the bugs using neem oil or perhaps trying solution of rubbing alcohol and water; top needs to be cut off until healthy tissue is visible. Rot spreads easily and will rot whole plant unless something is done ASAP. ps: is it hot & humid in your enclosed atrium?

Nothing wrong with it. You have the variegated and monstrose form of Opuntia monacantha. It may have been in the dark for a long time, and so lost its variegation. You probably put it in better light, and now you can see the white again.

Double post, already answered in the other thread :-)