They're on your tomato plant because their mama thought it looked like a tasty thing for them to nom on. Pinch off the leaf they're on and throw it out (in the garbage, not your compost).
Probably white fly larva. Spray the plant down with hydrogen peroxide watered down by about 3:1 at a minimum. Alternatively you can buy some oil based sprays (neem and others, iirc) that protect against mold, fungus, white flies, etc.
Yeah, a little dawn or similar in a spray bottle works as well. When you start seeing shit like this you need to take care of it asap. They can spread and destroy other plants and even neighbors plants.