Could be an aphid, could be just about anything but it looks like a leaf-sucking kind of insect - or a fungus gnat but they're usually black and tend to fly when they're out of the soil.... If you're concerned, grab some Dr. Zymes Eliminator or some Spinosad - both are organic and can be used, if necessary, straight through to harvest. Use as directed but spray at dusk or whenever the plant will no longer get direct sunlight... If you think it might be something living in the soil, Mosquito Bits is what you want to get... again, very effective at killing adults, eggs, larvae and you soak them in water, strain them out and then use the water to water your plants...
Good luck!