Greene: Thank you, I do believe you nailed it! Snow Squarestem (Melanthera nivea) is a Florida native; it's found all over the state and is a definite match to both leaves and bloom of the plant with the white flower.
Tiffany: False Buttonweed (Spermacoce verticillata) is not native here but is found in many areas of Florida; it reportedly blooms year round but the leaves don't match for the plant in question with the white flower.
I hope to visit the NWR at a time when that other plant is in bloom to try and get an ID on it.
