They are aphids and soap and water will deal with them effectively. See here for more details. For a bonsai you do not want to take the risk of burning the leaves so be sure to rinse the leaves a few minutes after spraying.
The key to control is to spray at least three times at five day intervals. This will catch the newly hatched young of the next generation.
Edit: either method works well. It depends on the number and size of leaves. Many small leaves would make spraying easier. Using a paper towel to rub the underside of the leaves does take care of the eggs as well but takes longer.
Edit: to remove the soap I would spray it with water. If you want to remove it with a damp paper towel you will have to do it a number of times to reduce the residue level.
Just remember: only 5 ml of soap to a liter of water, a little goes a long way
Edit: it doesn't really matter what soap you use. As you are dealing with a soft bodied insect all kinds of soap will do the job. All soaps are fatty acids and dish soap works just as well as "Insecticidal soap". Use the simplest plainest type you have.