Fenestraria aurantiaca is the classic strange succulent in the Mesemb family. Related to the Lithops, these are also very low water plants. We recommend keeping them out of full sun and watering every 3-4 weeks. With more sun and more water they can grow quite big, relatively speaking, but then they are very rot prone and most people will find that a higher water level schedule will kill them. Harsh!
The Fenestraria genus includes only two species: Fenestraria rhopalophylla (with white flowers) and Fenestraria aurantiaca (with yellow flowers), which in time have gained various hybrids, with very beautiful flowers (red and orange).
It also appears that F. aurantiaca is no longer considered a separate species, but is a subspecies of F. rhopalophylla. So I guess I better get all my tags updated.