Something from Rubiaceae. Could be really unhappy Diodia virginiana.
Yes, I know Hygrophila pretty well. And it's not that.Yes, plus I did some research today and the stem is square, the viens in the leaves are longitudinal with a serrulate edge. Here's a good link for some Hygophila facts, play the video http://plants.ifas.ufl.edu/node/191
Sorry about the photos but I was at risk of getting hit by a car!!