Hi Chris - 
   Nope! Your green mat algae can not be Didymo.  Didymo
is a diatom.  I.e., it is a one-celled algae that forms thick, 
dark-colored, repulsive mats.  
   The algae that you are referring to is probably Spirogyra
(or a close relative) that forms bouyant mats as a result of
sexual reproduction (conjugation) this time of year.
   Hope this helps.
Best wishes,
Lance