Not a cactus. Peperomia ferreyrae, an epiphytic, succulent from Peru. The shredded bark in the container also is consistent with this. It also appears similar to some of the Senecio species because of the waxy coating on the new foliage, but the older foliage has the characteristic stripes running its length that the Senecios do not.
A link to a photo of Senecio mandraliscae. http://davesgarden.com/pf/showimage/50206/ One can see where there might be some confusion.
