Context of the Foam

The author lets us know that this sea foam is toxic, but do we know what it is? Where it comes from? Is this a common occurrence? Or is this foam more of a commonplace at this beach?I would also like to read more about how local residents understand the coast and if, when, and how it is or isn't considered to be a safe place. How is it that kids (like those depicted in the image) are allowed to play here? Is this a widely accepted practice? Or would other residents find this equally appalling?