It sounds and looks like "scale insects". They suck on the plants and often just spread out on the stems or main leaf veins and still pesticides often does not work. I can´t tell for sure from the pictures you provided. So please google "scale insects" and compare these with your disease/pest. If your plant is infested by these insects you could mix soap (the one for the dishes) with water and "wash" your plant and the insects away. But please consider, soap mixed with water is not a proofed pesticide, another abiotic damage could folow. But I never had these problems.