From FrugalReef

Cyanobacteria (Aka. Red Slime Algae) is a purple, red, green or black slime that grows on your glass and rocks. It is a primitive single celled bacteria that prefers areas of stagnant, low oxygenated water. It's common right after cycling, but continuous problems with it indicate that you're not getting enough water movement. Try adding a power head focused on the area of effect.

This algae has a reputation for being unpalatable for algae eaters, but Cerith snails and Mexican red legged Hermit Crabs are known to eat it.

[edit] See Also

• Algae Control
• http://home2.pacific.net.ph/%7Esweetyummy42/conditions.html - Pictures