These things are a horrible nuisance. Try a soap based spray first. That won't harm the plant but should take care of the aphids. Just make sure you get top and bottom of the leaves and the trunk covered real well. If the plant is too big for a bottle spray, get it concentrated and put in a pump sprayer. Good luck.

I have those on my dwarf pomegranate and orchid flowers. I just use a water hose and spray them away. But they kept on coming back. :( Very annoying.