Moon Cactus is a great cactus for beginners. It grows well indoors, and thrives on neglect.
It is actually two separate cactuses grafted together. The top, colorful graft comes from Gymnocalycium, with the rootstock cactus coming from another cactus. The colorful Gymnocalycium is a mutant, unable to photosynthesize, and therefore requiring nutrients from another cactus.
3" size pot