Ooh...cue ominous music...Looks like watermeal. Can you take a few closeup shots of it? Or, if you've examined it closely, is it really fine, as in as fine as sand? If so, it's watermeal, and there's only one chemical that will kill it, Flouridone. The other option is to stock tilapia, which are said to eat it well and are recommended as a control option by several state agencies. Don't let it get any worse or you'll be looking at a guaranteed fish kill. If it is indeed watermeal, it's the worst weed going.
