They tend to re-occur in particular areas. Most blue or green pond scum is not toxic but there is no antidote. If poisoning is suspected, immediately emptying the stomach by inducing vomiting may help. You may be able to do this by giving repeated doses of hydrogen peroxide until the dog vomits. Cyanobacteria produce several types of toxins, but the most common one damages liver cells resulting in massive bleeding in the liver, and liver failure.
Animals usually die from a combination of shock due to sudden blood loss, and liver failure.
The only treatment is supportive care - lots of intravenous fluids and other things to help keep the animal alive. Unfortunately, most dogs die. A pint or more of the concentrated scum, and maybe a quart or two of the water should be submitted to a laboratory qualified to identify the algae and detect the toxin. These samples must be prepared and handled correctly so you need to work with your veterinarian. Sometimes the identification can be made from samples of stomach and intestinal contents.