We've been calling these "white fuzzies" but haven't a clue what they are. This one is on an Echinacea angustifolia, and they may be found on other prairie plants. This particular specimen is in our common garden, a former alfalfa field containing E. angustifolia, Bromus inermis, Carduus sp, Heliopsis sp, Poa sp, a few Solidago species, Hesperostipa spartea, and numerous less abundant species from several families. As far as I know, we haven't seen these on other species, but we also haven't been looking, so it's entirely possible.