These little plant suckers are often not noticed until their numbers are out of control. Often they will also be seen with ants that are “herding” them for the honeydew the aphids secret.
One of the many in the aphid superfamiy Aphidoidea.
Aphids suck the goodness out of the plants and this leads to them being in poor health. The honeydew they secret attracts ants and fungal diseases and the holes they put in the plants can let in viruses.
You can also use bio-oils or a good spray with the hose pipe will knock them off the plants.