To me that just looks like the plant is getting older and is starting to create a kind of "bark" like a tree.
i dont think its anything to worry about, but to be sure try to scrape one off and throw it under a microscope. if it has tiny legs, it might be scale.
Ive never seen scales but after reading the scales description in this thread and how it can lead to black fungus on the leaves, and seeing your pic above^^^^. See how it looks like black spots on your leaves? Is that "sooty" mold or something else? I'd suspect Baba G hit it on the head. Not familiar with it tho. Hope you figure it out. GL. -PDX