Certified GKH Gardening Expert
That is a severe case of scale, and will be VERY difficult to get rid of. Most of the treatment will be, manually, scraping all of it off to the best of your ability. Then you can treat with organic or chemical means.
These articles will help: https://www.gardeningknowhow.com/plant-problems/pests/insects/what-is-soft-scale.htm