Certified GKH Gardening Expert
While I cannot be sure, it sounds like a scale infestation. Neem oil should help. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
It could also be lichen, which is not harmful to the tree but is attracted to unhealthy or stressed plants. If you want to get rid of it or simply get more info to see if this is your problem, here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm