Yes, aphids.

They commonly attack new plum leaves each spring but are gone within 4 to 6 weeks. Then ensuing growth will be normal.

Overall the curled stuff isn't significant to the tree's health.

But if you don't like the way it looks, every spring keep an eye on the tree and, at the first sight of aphids, wash them off!

I don't use any oils or pesticides for insect control as I grow organically. google Arbico. they specialize in beneficial insects. I ordered three praying manti egg cases. one for my yard and for my neighbors on both sides. one egg case hatches abou two hundred manti. They devour every insect you hate. and once they live out their life cycle they lay their own egg cases. so now I have an ongoing population of manti. I live an hour north of death Valley so its a desrt hot blazing summers freezing winters if my manti can stay alive year after year your area will support them as well. also consider Ladybugs these are literalyy the toughest bug in the pack they eat every single pest bug you can imagine problem is when they eat everything they move on to find more food. I even take my baby manti indoors to work over my bonsai and houseplants I leave a couple of windows open all the time for my cats to get in and out. so manti have an escape, or entrance if they choose. I grow vegetables, tobacco, fruit, housplants and bonsai. this was a permanent and inexpensive solution to my bug problem. Good Luck!!!!