Over 20,000 species known to exist. Algae may form on your pool surfaces or it may bloom in suspension. We typically know algae to be green but it may also be yellow (mustard algae), black, blue-green, or any shade in between. It may form separate spots or seem to grow in sheets. Pink algae are not algae at all but a form of bacteria. Algae are living breathing organisms that need warmth, sunlight, and CO² to thrive. Because of their effect on pool sanitation and appearance, their presence is more objectionable than pathogenic.