Aphids On Leafy Vegetables

These look like aphids. They will typically feed on the newest plant growth. You can identify Aphids (with a magnifying lens) by the two “cornicles” (structures that look like tiny tailpipes) extending from the rear. If you see the cornicles, you would want to control these insects with a pesticidal soap if this plant were growing outdoors, available at your local garden center or wash off with a strong stream of water. However, in a greenhouse this is a nightmare and you should destroy the plant by taking it outside into the cold. These insects reproduce rapidly (giving live birth, rather than laying eggs), so you can also narrow the possible insect species down if you see that some of them are smaller versions of the larger ones. Sometimes an adult will develop wings and they look a bit different than the rest, but they’ll still have the “tailpipes”. Also, the white bits are probably the cast-off exoskeletons of aphids, while the green or grayish moving creatures are the aphids themselves.Although the white bits are out of focus and may be whitefly. Here is a link that you might find helpful:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html