It can be a sign of the maturation process of your tank- as it cycles, the algae feeds on the nutrient byproducts. Just scrape it off, and keep an eye on your parameters- ammonia, nitrite, nitrate, etc. It also arises from a lack of circulation and subsequent removal of wastes, and can be a consequence of lighting. If you have sufficient flow, good skimming and do regular water changes, the algae will disappear when the tank is fully cycled- good luck.