Dave,
That growth looks very much like the filamentous algae that I am familiar with on the fresh water lake at the nature reserve that I help to warden here in the UK. I would normally associate it with warmer sunny conditions and some sort of nutrient source that enriches the water and raises the levels of phosphate and nitrate in the water. Low water flow rates that fail to flush the nutrients away would also contribute to the growth of this stuff.
Barrie