It would be helpful to have a photo, and also to know what part of the country or climate zone you are in. What comes to mind is that they are aphids. Are they roundish and in groups clinging to stems and leaves? Aphids are sap suckers mostly planted by ants. Are there ants on the plants as well? This is a long shot guess without photo or location but this is what comes to mind from your description. You can get rid of them by spraying a mixture of dish soapy water on the plant, morning or evening when the plant is not in full sun. A couple of hours later you can hose the dish soapy residue off of the plant along with the dead aphids.