The ruby ball cactus, also known as the red cap cactus or the moon cactus, is a grafted specimen. The colorful red top (the sicon; is Gymnocalycium mihanovichii - the name refers to the flower buds bearing no hair or spines). The lower green cactus host portion can be any number of species, but is usually a Hylocereus cactus.
Like many cacti, these plants prefer a drying period between waterings, even to the point where they slightly wilt. When you water, however, you should water deeply. The plant will noticeably plump up. It is imperative that the cactus is not exposed to prolonged dampness and standing water, which can cause root rot to develop. A rich, fast-draining cactus mix with a low pH is ideal. Look for a bright area, but not so bright that the color of the top begins to wash out.
There are many unique varieties of Gymnocalycium mihanovichii cacti, as these are collectors' plants with a wide following. The appearance can vary widely, depending on what species within the Hylocereus genus has been used as the host, and on the appearance of the upper sicon, which is generally a mutated strain of various Gymnocalycium mihanovichii cultivars. They can be red, orange, purplish, yellow, or even white. Older plants sometimes flower with pink blooms during the summer.