Thank you for your question. There are 19 insects that are pests of plum trees, listed here, about half of which are capable to eating foliage:  https://pnwhandbooks.org/insect/tree-fruit/plum-prune

We cannot identify an insect without a clear, closeup photo of the insect, as well as whatever damage has been done.    If you'll send us some photos, we'll try to be specific and suggest control solutions.  Thanks!

It appears I have aphids after looking at the bottom of the leaves. Let me know what you think. How would you advise me to tackle this pest?

Here is information on their life cycle and their natural enemies: http://pnwhandbooks.dev.extension.oregonstate.edu/insect/hort/landscape/common/landscape-aphid If they can be reached, aphids can be knocked off with a strong stream of water. Squishing some will often cause others to jump off. Horticultural oils and insecticidal soaps help when thoroughly applied in the dormant season. One ongoing method of control is to remove leaves and other plant tissue in the fall. Monitoring for them with sticky tapes is necessary in the spring. I hope this is helpful. Good luck!