...I also found a green plant that I initially thought was a different color of Ludwigia, because it was growing among the red stuff and had very similar leaves. On closer inspection, though, I see that the leaves are alternate rather than paired like the Ludwigia leaves.
Is there a common eastern U.S. aquatic species that fits this (admittedly vague) description? Since it had recently rained heavily, I suppose this could be a terrestrial species that was temporarily submerged, but it was definitely growing in the same locations as the Ludwigia and Proserpinaca.