What is this on my Pachanoi ? thanks in advance.

Hi everyone, I’m looking for some advice. I grew these Cereus repandus from seed five years ago.

This dark grey/brown growth/discoloration covering the skin getting worse since one year. Does anyone know what this might be? (e.g., fungal, scale, or corking?) I've attached photos for reference. Any tips on how to treat it would be greatly appreciated!