Blue Green Algae is No Joke

"Any of the cyanobacteria toxins can be harmful to people and dogs, as well as other animals, and these toxins have no known antidotes.

"Some cyanobacterial blooms can look like foam, scum, or mats that lie on the surface of fresh water lakes and ponds. Some blooms may not affect the appearance of the water. The highest concentrations of toxins are usually found in blooms and scum on the shoreline. These dense accumulations pose the greatest potential risks to people and pets.

"If the blue-green algae bloom produces high volumes of toxins, then touching it can cause rashes and other skin irritations as well as itchy nose, eyes, and throat. Ingesting it can cause diarrhea and vomiting. At high enough levels, toxins can cause liver and nervous system damage. In the case of small children or pets, due to their body mass, the toxins can be fatal." 

The article says to wash off blue-green algae with tap or bottled water, avoiding touching it. Don't use bleach on it as this may crack open the algae cells and spread the cyanobacteria even more. 
http://www.dogheirs.com/dogheirs/posts/306-blue-green-algae-cyanobacteria-and-how-it-can-harm-your-dog