Yes, its dying. No, no way to save it.
Moon Cactus, by their very nature, are living on borrowed time, no matter how well you take care of them.
Moon Cactus are two cactus varieties grafted into one plant. The bottom part (Hylocereus) is supporting both itself and the colorful ball on top (Gymnocalycium). Its a parasitic relationship with no chance of either partner surviving. Sorry!