These group of red algae is called Rhodophyta. They contain about 6,000 species and many are marine seaweeds.
They are red because of the pigment called phycoerythin, which absorbs blue light and reflects red light.
Their coloration, which depends on how much of this pigment they have, ranges from reddish yellow to bright red to greenish blue and brown.
Because blue light penetrates much deeper than its color, these algae usually live at deeper depths and are well adapted to low lighting conditions.
They are introduced into the saltwater aquarium on live rocks.
The most common are called coralline algae, which secrete a hard calcareous shell the way corals do.
They are also considered to be important in the formation of a tropical reef.
They are beneficial to your saltwater aquarium, encrusting rocks and even spreading to fixtures and glass.