Water meal, native to Florida, is a tiny, floating, rootless plant. At 1 to 1.5 mm long, it is the smallest flowering plant on earth. It is occasionally found growing in rivers, ponds, lakes, and sloughs of the peninsula and central panhandle of Florida (Wunderlin, 2003). Water meal is barely discernible to the naked eye and occurs almost always (estimated probability 99%) under natural conditions in wetlands.
R.K. Godfrey says he "sometimes tests for its presence by taking a handful of whatever duckweeds are present and rubbing them between the palms of the hands. If Wolffia is present it gives the feeling of granularness or mealiness."
For brief control information, see Efficacy of Herbicide Active Ingredients Against Aquatic Weeds by K. Langeland, M. Netherland, and W. Haller.