It looks like a form of corking. This can happen in response to a variety of things, sometimes it is just age, but that tends to not spread quite so heavily towards the top of the plant in my experience. I have associated it with longer term low(ish) light conditions for plants that like to see more bright light, but unfortunately there is also such a thing as corking disease which leads to uncontrolled corking all over the plant. I do not know how to treat that except to suggest try to do things that might improve the plants growing conditions: new soil, better light. They idea being that new healthy growth will hopefully stop and outpace the corking.
I have a plant (an Echinopsis/Trichocereus hybrid) that started getting it all over its main trunk, when I moved it from its pot to the ground, It started growing more vigorously and started making new arms. The arms all are healthy looking while the main trunk is still just outpacing the corking.
