Q.Fairy Cactus
Why is my Fairy Cactus splitting vertically?
Certified GKH Gardening Expert
This is normally caused by overwatering. You should make sure that the cactus dries out thoroughly between waterings. This article will refresh you on the care of these cacti: https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
The first thing that comes to mind is root rot from over-watering. Your cactus should be in potting mix designed for cacti. If this isn't available, mix one part perlite and one part sand to 2 parts regular potting mix. (Other recipes are out there; the point is to get a mix that drains freely and doesn't get soggy.) Let the plant completely dry out before watering to minimize the risk for root rot. I suggest you take it out of its pot and examine the roots for soft, discolored roots. Prune out those roots. Repot into a terra cotta pot which will help the plant dry out, further decreasing the risk of rot.
General information on fairy castle cactus: https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
