Ken at rhipsalis.com calls this "gray crud" which shows up on rhipsalis. Ken, Derek and I have exchanged some emails about it a while back but come to no firm conclusions. I think that there are some things that might be possible.
1) This appears commonly on rhipsalis, typically older stems. Over time, it becomes thicker and almost bark-like on older plants. In some cases, this may be normal growth of the plant as the stem ages or a response to stress.
2) It also may be mineral buildup from spraying with other than distilled or rain water. Rhipsalis (mostly) grow in highly-humid environments. We mist them to compensate, but the water used to mist contains minerals which can build up over time. When the plants grow outdoors, the rain can flush some of this away. The whole plant needs to be watered with very soft water to prevent mineral buildup. This is very difficult to do indoors, unless one has a greenhouse. Ken and I have experimented with our plants. For me, I seem to be able to reduce or prevent much of this by withholding misting during the winter and during the summer, I drench the entire plant when watering with the hose (using a gentle shower or mist setting). During the winter, I will occasionally put a few of the more stressed plants in the shower for a thorough, but gentle, shower with tepid water. The plants like this, but I dislike dealing with the mess it creates, so don't do it unless I feel a strong need. If misting over time is the cause, then that would also explain why this is more common on older stems.
3) Bacterial or fungal growth is also possible. I have tried swabbing affected areas with pure peroxide from a bottle. It did not seem to have an damaging effects to the plants. I could not determine if that stopped the progression of the crud because I had also adopted the practice of no longer misting my Rhipsalis.
This experimentation began last year. I find it interesting to note that my Rhipsalis have not been as healthy this year as in the past. I think this is because of the dry air indoors and not misting this winter. Next winter I am going to try putting plastic bags around some of the ones that did not do so good this winter.
Not misting has seemed to reduce the gray crude propagation. However, drier plants mean a less comfortable environment for bacteria and fungi, as well as less plant growth, so nothing is conclusive about the cause, just something to try to reduce the crud increasing.
So, if the gray crud is a problem and you have been misting, switch to misting with clean rain water or distilled water. Alternatively, periodically give the plants a longer term shower rather than just misting.
BTW, I've learned this winter that brown crud on Rhipsalis can be caused by exposure to too much light. Some Rhipsalis turn red when exposed to bright light. Some of the ones that I have that get a lot of light and do not turn red have developed areas of brown crud under bright artificial fluorescent light. This includes R. sulcata. (low humidity, bright light and not a lot of water)
Hope this discussion helps. I can't say for sure what causes the gray crud, but try the suggestion above and see if it helps. Let us know if it works for you.
-t
