Night Blooming cereus
What does it mean if the leaves on my night-blooming cereus are turning gray?
Certified GKH Gardening Expert
This is a fungal infection. Usually this is caused by overwatering. You should only water when the top two inches of the planting material is almost completely dry. You can attempt to correct the issue, as long as it isn't too far developed. Mix peroxide in water. Just replace 1/4 of the water with peroxide. This will kill off anything that doesn't belong there.