I noticed these small (less than 0.5 inch) clusters of water fern when I was photographing the surrounding duckweed (Spirodela sp.). Apparently (acc. to ITIS.gov) all the water fern occurring across the southern U.S. is now known as Azolla filiculoides, having previous been known under the names A. caroliniana (e.g. North-Central Flora) or A. microphylla (which BONAP still retains).