This didymo stuff looks and sounds a lot like the algea which forms annually all over the North East River and Susquahanna Flats. Generally, the stuff is associated (up here) with areas of the water which possess large areas of SAV. It seems to grow in thick mats on top of the floating SAV where the water is calmer (due to the SAV absorbing the waves). It is emerald green with thick hair-like strands. I first noticed it in my teens while working the local marinas (about 20 years ago). It has steadily increased ever since. It is absent in the spring and seems to peak around now.
 
In case you aren't familiar with the North East River and the Susquehanna Flats, these waters, though tidal, are extremely fresh with only trace amounts of salinity. In fact, when it is tasted, there is NO noticable salt in it... 
 
If anyone would like, I would gladdly send a photo of it.
 
let me know if someone wants to try to ID "my" stuff...
 
-Chris Starling
North East, MD
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