This glass is filled with a type of cyanobacteria (also known as blue-green algae) that secrete toxins into water. This stuff can make you—and any animals that drink the water—very sick. It causes nausea, vomiting, diarrhea, eye irritation, earaches, sore throats, and more. Prolonged exposure can cause liver cancer. (Photo courtesy of Citizendium.)
If you live in the USA, it's pretty easy to avoid cyanobacteria. Don't swim in, or drink, water that has algae blooming in it. People in a developing country might not be able to avoid contact with blue-green algae. Not everyone is as lucky to have fresh water taps in their homes as we are in the USA.
See WHO if you want more information on cyanobacteria.