Filamentous algae are single algae cells that form long visible chains, threads, or filaments. These filaments intertwine forming a mat that resembles wet wool. Filamentous algae starts growing along the bottom in shallow water or attached to structures in the water (like rocks or other aquatic plants).

If an aquarium is not maintained properly, and in some cases even when it is, algae can run amok, choking out your corals and growing over your beautiful  live rock. It may take six months to a year before the big blooms come.
 

The most important factors affecting algae growth are light, oxygen, temperature and nutrients (e.g., phosphates, silicates, iron, dissolved organic compounds [DOCs]). It would seem logical that if we could limit these growth factors we could prevent undesirable algae from overrunning our aquariums.

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Solutions​​​​​​​​​​
Filamentous Hair Algae is not palatable by most reef herbivores,
Dipping the rock and/or coral in RPS ALL OUT will also  prevent an outbreak. Once you have dipped the coral you should place it in quarantine.

Sources
http://www.fishchannel.com/fish-health/saltwater-conditions/algae-problem.aspx