i just checked it. The soil was completely dry so I gave it some water and flowering plant food. Any advice is appreciated. The stem of the yellow cactus is rotting. The white spot in the photo is now considerably larger. Can anyone give me the correct name of the coloured cacti? Thanks

Ah, I'm so sorry. I had a ruby ball ( red top) and a moon cactus ( orange top) and lost them to rot. I'm still a novice, but this has happened to me. I wouldnt take my advice without getting an opinion from a more experienced cactus person or googling several sites. But it might be helpful to share my experience, FYI. in my case the rot was at the bottom in the roots .., I cut the bottom half off with a sterile sharp knife, dressed the wound with sulfur powder and let the cactus dry on a dish for 3 days, until it got a callous. Then I put it in new soil... It did fine for a month and then died. Now I'm not sure if what I did helped , because the problem was my apartment was very moist and dark..,I've since moved to a dry apartment on flood stilts ... I haven't had a casualty since.
Btw the cacti with the colored tips are actually two cacti grafted together the colored one is a mutation that has no chlorophyll and must live off the bottom cacti's chlorophyll. The bottom cacti is a tropical cactus, the top a desert cactus... So it's very hard to take care of these guys,IMO, from the start.