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You plant looks as if it may be a Melanthera. There are four species found in Florida. It is not Melanthera nivea, but perhaps it is one of the others: Melanthera parvifolia, angustifolia, or ligulata. Photos of those three are hard to find. I may be completely wrong, but the flower structure reminds me of Melanthera nivea, hence my guess that it is in the same genus.
@Darwiniensis: I will revisit the site and do some checking on the leaves to see if they match the description I found... looks quite like a young Palafoxia feayi. Thanks for the tip!
@meltonw: My original thoughts were leaning towards Melanthera, but the stem is round and the whorled leaves were throwing me for a loop. I think I did find a M. lingulata at the site, though... a bit out of its range.