Whereas green algae is good and "natural". From that article:
They're reducing the number of good phytoplankton in the lough that would normally compete with the nuisance phytoplankton," says Prof Mark Emmerson from Queen's University Belfast."That's releasing them (blue-green algae) from competition for limited resources which are nutrients, and they're blooming.
In the case if the article it's because of an invasive species (mussels). In the reflecting pool of there is blue-green algae, not just the good old green kind, it's as likely to be due to their efforts to kill of the green algae as anything. And if they do succeed in killing all the green algae they will likely then have blue-green algae or that red algae instead.
Water+nutrients= life. In nature that life forms a balance, you get a food chain and nothing gets too extreme. Fuck around (create a pond with no native life that eats algae etc/introduce invasive species/try to kill everything with chemicals) you get population explosions and ever worsening situations.