How to get rid of this particular plant pest?

My plant currently has some weird bug. I currently take care of some small tree and it looks as though it has a few scabs of some sort. The thing is, I can easily scrap off these "scabs" and the tree would look okay. I looked under a microscope to see what these scabs looked like close up, and I found tiny little pests crawling around in it. It's definitely not aphids, so what are they and what can I do to kill it? It definitely seems to be killing the tree as these "scabs" seem to absorbing sap.