Cactus in need of help

Hi all,
First time posting here and I've got a cactus in a bad way.
I suspected it was severely underwatered so repotted and checked roots for damage etc then gave a goot watering after a few days hoping it would perk up.
Unfortunately it only seems to have got worse, it looks like it's grown mould all over the surface (pretty sure it's not mealy bugs since there are a lot of other plants around with no issue at all).
The cactus still seems firm the stem isn't mushy at all just this external growth, which sort of rubs off but just ends up coming back again within a day or so, I've tried rubbing hand sanitiser on it since rubbing alcohol impossible to get hold of these days, not sure if that's a good idea or not...
Is there any salvaging this or is it a lost cause?
Any advice is hugely appreciated I'm pretty attached to this guy!
Thanks, Tom

 

Actually looks like a Euphobia.
If the white stuff rubs off, I'm tending to think that the damage was done by the hand sanitizer.
Unless of course it was going brown under the white covering already.

The main stem looks a lost cause so maybe you could reroot that side shoot.

 

Yes it's euphorbia ingens I believe, the hand sanitiser was a bit of a last resort I'd just wiped it with a damp cloth previously but after it came back I thought alcohol might kill the fungus and hopefully not the plant.
The brown is actually just the layer immediately below the outer surface, I cut away a small section from one of the spines and it looked perfectly healthy underneath I'm just not sure I can cut away from the flat part of the stem.
In terms of re rooting is it just a case of leave the end to callous over for a few days then plant in fresh soil like any other cutting?
Thanks

 

To be honest I don't grow any succulent Euphorbias so a cutting would be experimental for me as well.
Having said that, I do think it's the same approach as with cacti.

Sounds like the damage is only on the surface then, but it has ruined the look of the plant.