'Carolina mosquitofern', 'Carolina azolla', or 'water velvet'

Azolla is a genus of floatoing plant from the east coast west to Wisconsin and Texas, and in the Caribbean, and in Central and South America from southeastern Mexico (Chiapas) south to northern Argentina and Uruguay. It is a freshwater aquatic fern, with scale-like fronds 510 mm long, green to reddish, most often reddish in strong light and in winter. They are covered in tiny protuberances called trichomes that give it the appearance of velvet. It is able to fix nitrogen from the air by means of symbiotic cyanobacteria. It can survive winter water temperatures of 5 C(41 degrees Fahrenheit), with optimum summer growth between 2530 C. (77-86 degrees Fahrenheit)

The only sure method of distinguishing this species from Azolla filiculoides is to examine the trichomes on the upper surfaces of the leaves. Trichomes are small protuberances that create water resistance. They are unicellular in A. filiculoides but septate (two-celled) in A. cristata - Wiki

Evrard, C.; Van Hove, C. (2004). "Taxonomy of the American Azolla Species (Azollaceae): A Critical Review". Systematics and Geography of Plants. 74 (2): 301318.