TOXIC ALGAE BLOOM may have contributed to the deaths of a northern California couple and their baby. What is TOXIC ALGAE BLOOM?

"Harmful algal blooms are the rapid growth of algae or cyanobacteria that can cause harm to people, animals, or the local ecology. Harmful algae or cyanobacteria can look like foam, scum, paint, or mats on the surface of water and can be different colors. These blooms can produce toxins that make people and animals sick. Blooms occur in fresh water, such as lakes and rivers, and salt water, such as oceans or bays."

This article explains it very well and provides some pics.
https://www.earthday.org/how-toxic-algae-blooms-work/

The algae are poisonous - but only if you ingest them. A common way is simply poor hygeine.