The red cactus that’s grafted onto the green cactus can’t survive on its own even if it does manage to grow roots of it’s own it’s doomed as it can’t photosynthesise. Looks like the soil is peat heavy and too wet so the root system has rotted causing it to yellow from the base up. These cacti aren’t particularly long lived even in expert hands as far as I’m aware. Can see some perlite in the mix but there should be at least 50% or more and watered sparingly in good light. This looks far too wet.

Gill

That makes me sad. I was told the red is a Gymnocalycium. I was really hoping to be able to save it. I read about the moon cactus not having a long life span, which sucks cause its beautiful. Like I said, I just wanna save the red....or at least try.
As for the water...I asked my cousin to NOT water this one, and to water the rest of my plants.... he did it backwards, water this one and none of the others...and he gave it way to much water.
Even if it doesn't make it, I could still 'experiment'...? try and get it to grow..?
I mixed half perlite and half cactus soil.

Agree with Gill; it must have been overwatered/kept too wet for too long for it to start rotting. Base - the rootstock, is much more vigorous plant than one grafted to it.

IMO the only thing you can try is to take it out of soil ASAP and cut off all rotting part from the rootstock. Let it callus for few days, and pot into much more grainy mix. If you have only soil, mix more than 50% of coarse perlite with it. Keep it dry until roots start to grow. They may or may not - you never know unless you try.

I will definitely do that. thank you.
Would adding more perlite help with coarsness.... or do I need to buy gravel or something?

Rinsed perlite is perfect.

My recommendation is to allow the cut to callous for at least a week since it's so cold still.

Unfortunately I am sure :/

Oh, yup. Time to chop.

Unfortunately this spot doesn't look so good, either.

Or, maybe it is just the light. I don't recommend you try cutting that off. Just keep an eye on it I guess.

I used my flash...so it could be lighting.... I will double check in the morning. I have already cut off the really bad stuff...it is starting to dry now.

What would cause this?

@James Ball,

Sorry, that is rot. The way it is all up and down the side, unfortunately I don't think the top is savable. I guess if you are the adventurous type you could try quickly reading up on grafting, make a fresh cut on the top of the rootstock, which would simultaneously remove the rotting scion, and try grafting one of the scion's pups on to the fresh cut on the rootstock, if a good sized pup is available that the rot hasn't reached. Or, if you'd rather not try that you could just cut off the rotting Gymnocalycium scion and continue to grow the Hylocereus rootstock. It is an epiphytic cactus.

Thanks