It may also pop up if something goes wrong in the aquarium and there are large amounts of nitrates being produced for an extended period of time. Once an aquarium has finished cycling, this type of algae tends to become extreemly unnoticeable, or even dissapear altogether. But like most things, this is just a tendency, and not a rule. There are many animals that can help control filamentous algae. Fish, echinoderms, crustaceans, some mollusks, and some analids will eat filamentous algae.
Tangs are an excelent fish for controlling any algae, some pigmy angels, and many others.
Hermit crabs, decorator crabs, arrow crabs, and some species of shrimp are useful. There are a few species of nudibranchs that will feed on filamentous algae. I have even observed some analids such as fire worms eating filamentous algae. It is most often than not, unavoidable in semi-new setups, but with patience and proper aquarium husbandry, it will eventually go away. A species of Chlorophyta.