Answer: I'm not sure what the green bugs are but they sound suspiciously like aphids, soft bodied sucking insects that exude a sticky substance called honeydew when they feed. That's probably the sap you are seeing. Aphids can be controlled with insecticidal soap sprays but it can by phytotoxic to the leaves of some trees so read the label to make sure you understand when to apply the product. Once you get the aphids under control your plum tree should recover. Repeat sprays will probably have to be made. Good luck with your plum tree!
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