No, it is not polution. It is clay containing suspended particles of decayed vegetative matter. Because of the water plummeting over the Falls, especially the Horseshoe Falls and stirring up the clay below, much of this clay floats to the surface and sinks back down quickly. It’s just that the Falls never stops churning up the water, so it appears that it never leaves. This “sludge” is actually mostly from the shallow eastern basis of Lake Erie.
Here is an example of the clay that bubbles to the surface below the Canadian Falls. The power of the falls crashing down over the brink dig into the clay at the bottom of the Falls. Air bubbles are added to the loose clay and it floats to the surface.
The clays floats for a while then returns to the bottom, but because it is a never-ending process, there is always clay on the surface. It is harmless.
With the hotels and other attractions overlooking the Canadian-Horseshoe Falls and the powerful lights glowing over the Falls as they do every evening, it’s a wonderful time to visit Niagara Falls.