It looks like scale insects to me. The insects live under a hard little shell so most treatments don't work. If its a small tree, use some rubbing alcohol on cotton balls to wipe them off then wash the tree with warm soapy water to get rid of the alcohol residue. The good news is that they spread very slowly but you don't want to leave them there because they will spread and they are sucking the life out of your tree.
Daisy
