I agree with ewest, this looks like eugelena. Eugelena is usually associated with livestock or geese, or some sort of organic inputs in the watershed. Like tranquility referenced, it usually comes and goes with the rain, and I don't think herbicide does much to it.

Fisheries Biologist
Mississippi Department of Wildlife, Fisheries and Parks

"Success is often overlooked because it dresses up in overalls and disguises itself as work."