Can you tell me what’s wrong with this moon cactus? It suddenly started to change its colour please tell me what to do!

[Praneeth]

 

[Mike Allen]

Welcome to the forum.Zigs is the forum cacti expert, so he will tell you more. I have grown cacti and I'd say that your plant is grafted. Best wishes.

 

[Tetters]

Welcome @Praneeth . Sorry, Mike is right, and that cactus is grafted. The cactus on the top has no chlorophyll and cannot photosynthesize for itself. The green bit on the bottom is unable to support the top as it grows and it is doomed to die.
We have hundreds of cactus here - Zigs is out tending them at the moment. We never entertain the ones that have been grafted.

 

[zigs]

Looks like it's grafted onto Dragon fruit which is looking like it's suffering. The cactus is trying to put on extra growth which is taking the energy out of the rootstock. Not sure about giving it more sunlight as the cactus might get sunburned?

 

[cpp gardener]

Does the pot have a hole in the bottom?
How often does it get watered?
How do you determine when to water and how much to give it?