I agree...GHA is a product of excess nutrience in the water. It can come from many sources...food, waste, source water, etc. You now have two issues, first is finding and correcting the source the algae and the second to rid the tank of the exsisting algae.
IDing the source starts with testing for NO3
as mentioned. Next evaluate your source water, stocking level, feeding schedule, bioload and filtration. Most often manipulating these variables will help solve the problem. Added flow and replacing the aquarium light bulbs can also help depending on what you currently have.
As far as removing the existing algae goes, manual removal tends to work the best. At water change time, scrub the algae off of the rocks in the water you have removed from the tank. Do several water changes using high quality RO
water as your source for mixing. It does take a bit of work but if you stay ontop of things you should overcome this problem.