Yup, picture isn't very clear, even when enlarged, but I believe they are scale insects. They are insects that are mobile when young, but then attach themselves to a spot on the plant and don't move any more. They are bad news, especially when in a serious infestation like yours. They are sucking the juices (the life "blood") out of your tree. And they may excrete excess sugars in a sweet sticky substance called honeydew. The honeydew can foster the development of black mold, attract ants, etc.
You need to get rid of them. Moisten a Q-tip with rubbing alcohol and touch each individual insect with it. After you have done that, they should be easy to scrape or even wipe off the tree. This will get rid of all the sessile adults. It does not get rid of the crawling juveniles, so the treatment will have to be repeated every couple weeks for awhile.