Blue-green algae isn't actually algae, but a bacteria called cyanobacteria that are normally present in many lakes. The bacteria thrives in warm, stagnant, nutrient water and when it thrives, it forms quickly.
Sometimes the algae look like grains of floating green sand or scum. It's often described as pea soup or spilled green paint.
"Any time you see unusually colored water, or vividly colored scums, be aware that this might be blue-green algae, and definitely keep pets and small children out of the water when the water looks like this," said Laliberte, Wisconsin DNR.
The algae can go undetected by dog owners if they lurk under the water's surface or attach to plants. Wind can also blow algae from one area into another that had previously looked clear.