Help my cheese plant is going black!

[Albadross]

Please does anyone with knowledge on house plants know if this is root rot?

We just moved so it's been moved about a bit and was in a fairly dark spot under the stairs, so I moved it into my study with a bit more light but when I turned it round I saw this...

What do I do??

Pls forgive the wallpaper - decorating in progress

The black bit looks dead/damaged - it likes high humidity, warmth and shade. If is has got too cold, not had enough light, etc. that could be the problem. I'd cut that stem right back to below the black area.