The little yellow cactus on top is Gymnocalycium mihanovichii. They are naturally occurring variants but, as Sy pointed out, do not produce their own chlorophyll so must be grafted to another cactus, in this case, a Hylocereus. As the Gymnocalycium grows, it will demand more and more from its host plant. Eventually, the host plant will fail and both plants will die.
It's a parasitic relationship and nothing you can do will save them.
