These fine strands are filaments of the bacterium Beggiatoa.
The filaments can grow and tangle to form slimy greyish mats up to a metre across covering the substratum in polluted waters. They are especially common in waters close to sewage outfalls. Unsightly as they are, the mucilaginous secretion which binds the mat together also serves to capture and remove suspended particles from the water.
Additionally, its growth necessarily involves the removal of organic compounds from the water, so this bacterium is not only evidence of water pollution, but also a participant in the water cleansing process.