Green Hair Algae (abbreviated as GHA for this article) - Derbesia - is a form of algae that looks like its name. It is usually dark green, grows quite long if left unattended and spreads across your tank rapidly. Once this problem has begun, it can get out of hand in a matter of weeks. And soon you start to look at your tank in disgust, thinking “Why me? What did I do to deserve this plague?”
A number of things may have contributed to it getting a foothold in your system. Your nitrates might be too high, your phosphates may be too high (.03 or less is the goal), your lighting has recently been replaced or perhaps your bulbs are so old that the spectrum of light has shifted, fueling algae growth.
First things first. You need to get your water parameters to Natural Sea Water levels: