Stapelia gigantea, or Carrion Flower, Starfish Flower, and several others.
I have recently learned that his succulent-type plant is actually a member of the milkweed family. This perhaps gives reason for its vigorous growth, a habit well known among that family. The plant is really easy to grow and flower, though newly rooted cuttings sometimes must establish themselves a year or two before they will bloom.
Yes, those are sepals. The petals are the tiny doodads in the center.
As mentioned, the plant is a weed, and scrambles over the edges of a pot in no time. These flowers were all on one section of stem that was well over the edge of the pot and the basket in which it was sitting. Their combined weight got to be too much. You can see it breaking off below.