Chris and I went to Niagara Falls four years ago and I don't remember noticing any slime, this is the American Falls:
Possibly Lesley saw the same stuff as is presently everywhere in the Dover Strait as discussed previously, a seasonal algal bloom. A Google seems to corroborate this: "Although I doubt this is the reason for the clumps of foam that you see below the falls. What you are seeing is the result of decaying organics. The same algae plants and diatoms that give the water its fluorescent color also contribute to the grayish foam you see floating on it. The waters of the Niagara, rich in dissolved calcium (in the form of calcium carbonate, from dissolved limestone bedrock) react with the organics in the water (from decaying plant and animal material) creating a sticky substance. The turbulence from the Falls and the rapids creates foam out of it. The foam isn't very stable and it tends to dissolve within minutes, only to be replenished by more."