I barely ever water it, so it can't be over-watering.

Does the top HAVE to be grafted, or can it root on it's own if I let it?

red top probably retains enough genetic preprogramming to remember how to grow roots if you let it callous over (a week or 3, doesn't hurt) then set it on suitable rooting medium & let it do its thing. It's got enough stored energy to survive a fair while.

But in the long run its got no green to help it photosynthesise more food & unless it can revert to green it'll eventually starve.

No idea if it's possible to shock it back to green, wouldn't mind experimenting... been keeping eye out for any neighbours abandoning their dying Xmas gifts but so far no dice.

I recently bought a small G.mihanovichi which is one of the origins these neon variants were bred from... this one is a dark purpley green & living on its own roots fine. Just 2" and flowering too, so must've been going least 5-7yrs.

The base - rootstock plant - is much more vigorous than top - the mutant without chlorophyll. That's the reason that after a while the top usually start declining. There are many discussions about this plant here, here is one running now:

-click here-

Variegated and the purple one Tamiya mentioned (Gymnocalycium mihanovichii) grow quite well on their own roots, the yellow, pink or others without any chlorophyll may live for a while but eventually die.

The rootstock is very vigorous plant and declines if over-watered. Maybe your soil is very water retentive, and while it may look dry on top, could still be wet inside-around the roots. There is also possibility of some bacterial disease...

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Rina, that's the thread I was reading after I posted this! Haha. I guess I will just let it do its thing. I'm not overly attached to it, so I'll see what it does. I'll consider grafting it if I can. Or I'll chop the top and let it root and live until it dies.