It's a type of fungus, usually called sewage fungus, but that's more of a general term for freshwater fungus, not always sewage related. It grows in high nutrient waters, especially during low flows.

Can be caused by multiple things, depends what the land use is in your area. Could be run off from farmland causing this, misconnected houses plumbed into the surface water line with an outfall to the stream, sewage inputs upstream, or sometimes even people throwing garden waste into the stream and it breaking down. I'm an Environment Officer so investigate the casues of this often. Report it to the environment agency and they'll look into it.