Hi- your plant is infested with aphids- there are many different species that feed on vegetable crops. You can sweep them off the plant into soapy water or remov entire leaves that are heavily infested. Horticultural oil and insecticidal soap will kill aphids if the they are sprayed directly. You can also wait a bit and look for predators, such as ladybird beetles, and wasp parasitoids to come and feed on the aphids.
https://extension.umd.edu/hgic/topics/aphids-vegetables
https://extension.umd.edu/hgic/topics/natural-enemies

Growing rhubarb in a pot is unusual. Be sure to keep it well-watered, add compost if possible to the growing mix, and keep the pot in a location that receives late afternoon shade.
Jon