The most likely cause has to be flow or lack of it. If you have any dead spots in your aquarium you will soon find them because this is where this dreadful pest will start. Little tufts of green strands will start to appear. Maybe it is time to upgrade your power head or maybe add another.
The second cause of that should not happen if you are doing regular parameter checks, is high phosphates. Simply resolved by the use of media such as rowaphos. It has to be said that this can be placed in the bag provided into good flow in the aquarium. Better methods include a fluidised reactor, although this creates additional cost but is a much more effective way to deal with the problem.
The final method i have read about is the actual lighting period, Too much of it will create this algae in uncontrollable proportions. Aquariums located near windows or other high light sources in the day are sure to have problems. So moving your aquarium or reducing your lighting time could help reduce green hair algae.
By simply blowing off your rocks on a weekly basis with a power-head will help keep this at bay and help prevent detritus building up.