Hi - It is difficult to see the details in your photos, but yes, these look like aphids (likely) or possibly a type of plant hopper. These both are typically minor pests that can be controlled by natural predators in the environment. You could hit them with a strong spray of water to dislodge them. https://extension.umd.edu/hgic/aphids-trees-and-shrubs

If this is a type of plum tree you are growing for fruit production, we do recommend following the the Virginia Tech Home Fruit Preventative Spray Schedule to manage common diseases and (more problematic) pests of plums, https://extension.umd.edu/hgic/topics/home-fruit-preventative-spray-schedule
The most common problems of fruiting plum trees are described here,
https://extension.umd.edu/hgic/topics/stone-fruit-problems
And here is our page on general care for fruiting plum trees,
https://extension.umd.edu/hgic/topics/care-fertilizing-fruit-thinning-harvesting-spray-schedule-stone-fruit

Christa