Some info I found:
The Ohiopyle bloom is located in the pool below the falls, a spot that is difficult for wading anglers to access but a popular launching point for kayakers.
"That area gets very little fishing pressure," said Dale Kotowski, a fly-fishing guide for Wilderness Voyageurs, an Ohiopyle outfitter since 1964. "I can't imagine the thousands of paddlers who come to that area, so it doesn't surprise me that's where didymo showed up first."
It's too soon to tell how the presence of didymo will impact Youghiogheny trout stocking and native trout reproduction in the region. The waters downstream from Ohiopyle are clearly in jeopardy. Considering didymo's documented propensity to spread quickly, all of the popular trout waters of the Laurel Highlands are also at risk, including Meadow Run, the Casselman River and Laurel Hill, Dunbar, Dunlap, Jones Mill, Indian and Blue Hole creeks.
The falls area at Ohiopyle fished fine in late April, he said, and to date didymo has not been detected upstream on the 10-mile Middle Yough from Confluence to Ohiopyle.
Link to source: http://www.post-gazette.com/stories/sports/hunting-fishing/didymo-algae-is-poised-to-foul-southwest-pennsylvanias-trout-waters-640706/