I am no pro but it looks like rot to me. I would pull off the rotting pup to see if the top part of the plant is okay. If it isn't I would hack it off so the bottom plant will survive. They are two separate plant. The top is grafted to the bottom.

A mutant gymnocalycium growing on a graft
The graft shows (as it looks) had been over watered and has signs of rot.

If you have the recipt bring it back

Wait... so all of those you see at like home depot that are like this are one plant grafted onto another?

How lame!! Poor plants..

To me, these are 'Moon Cacti" are temporary plants at best. I personally would not buy one, they just look too unnatural. The yellow cactus has been grafted onto another cactus, apparently Hylocereus, aka Dragonfruit, because it cannot produce chlorophyll on its own, and would otherwise die. I would remove the top section before the rot spreads. The green section may be salvageable, if the roots are not rotted. Did you keep the foil covering on the pot for the past 6 months? If so, the soil could not drain properly, probably leading to root rot.

Christopher

Much to protest regarding the many plants in big box stores though out this froum concerning the shady deals of big box stores.

Before you saw this one or even paid for it odds where not in your favor for sucsess.
Mutant plants, especaily with cacti of this type have only one small problem. Of the many things that can go wrong with a mutant plant in it's life any one of them things that can go wrong, will go very very wrong, very very quickly.

Pls. take off the foil if that's still on there after ALL this time. If it's not already rotted (sorry) it may help w/ the drainage.