I just recently saw a thread asking about a growth that looked identical to this. Nobody there knew what it was either. I've certainly never seen anything like it.
I hope somebody comes up with a definitive answer to this puzzle.
I found 1 like search on wet web media that looks the same. For some reason I can't link it (might be my phone). Reading the article it's more about how to control algae then about what that is. Someone replied with it's a type of black beard algae which in reality a type of red algae.