Plum tree parasite?
Picture provided. What is this?
Certified GKH Gardening Expert
It could be leaf galls. They can be caused by pests or disease (typically pest), but other than looking a bit odd, they do not cause any harm. If you seem to be getting a lot of them and want to try to stop them, you can treat the tree with neem oil, which will kill the pests that create galls but is organic and safe for people, pets and beneficial bugs.
I would not recommend trying to remove them. They can be removed, but the damage the removal does to the tree is greater than the damage the galls themselves do. Here is more information: https://www.gardeningknowhow.com/plant-problems/environmental/treating-leaf-gall-on-plants.htm