Red algae is a natural organism, like any other plant. Overgrowths of red algae, called algae blooms, which are deadly to fish, are caused by pollution. The algae thrives in toxic waters harmful to other creatures. These waters end up low in oxygen, and massive die-offs of fish occur.
So pollution creates an imbalance that includes an excess of red algae. But red algae has its place in nature, like every other creature.
These are also called red tides. They can occur naturally, but are also caused by: excess nutrients (pollution containing protein or carbohydrate); heat pollution; and iron-rich pollution: http://en.wikipedia.org/wiki/Red_tide#Causes