Certified GKH Gardening Expert
It appears to be one. I would recommend using wettable sulfur and DOLOMITIC lime together to correct the issue.
If that doesn't stop the spread (old spots will not disappear, but new growth will be clean when it starts to work) then you may have to turn to a more harsh copper fungicide.
This article will help with the care of these plants: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm