The issues are with the species of certain algae. Some of them
have endogenous chemicals in them as part of their makeup (not pollutants), that are toxic to mammals.

This is complicated and you just have to read the data and newest research to see what species are involved. Hopefully (but I see no guarantees yet) that the types grown in the vats commercially are not involved, or that the purification and processing remove the toxins like BMAA and the microcystins that are explained in the Wiki article.

There are algae today in lakes and drinking water, carrying these toxins! Heating and boiling the water releases them MORE (by bursting the cells), so is not a solution. Last year I read some papers online about the Great Lakes going back decades! It is just the public was not really aware of this research going on.