What kind of algae ?

[FoFrog]

 

[TheShrimpNibbler]

Cyano?

 

[Scrubber_steve]

Red slime algae is actually not a "true" algae at all; it is a bacteria that is technically known as cyanobacteria. Often considered to be the evolutionary link between bacteria and algae, cyanobacteria are among the oldest forms of life on earth and date back at least 3.5 billion years. These organisms produce oxygen as a byproduct of photosynthesis, and scientists believe that if it weren't for this microscopic organism, there would be no blue skies on Earth.

Aquarium Red Slime Algae Causes and Solutions