At the bottom of the algae page for Dennis Wong's page about algae, there is a option to get a algae control chart. If you're interested, sign up for the chart. I've done it and haven't heard from him since. He doesn't load up peoples emails with spam and other junk.
On that chart, it talks about all kinds of algae, including green spot.
Stressed plants or plants that are in poor health
High organics in the water column
Possible insufficient amounts of phosphates - this applies if green spot is the only algae present
Address plant health issues
Clear organic waste - Pruning plants will help with this along with surface vacuuming, regular filter maintenance, ect...
You haven't said if you're dosing your tanks or not. If you're not, you should be. If you are, posting what you're doing would be helpful. There are test kits to check phosphate levels.
Pick one solution and give it a few weeks. If simply reducing organics in the water column fixes the problem, you're good. If not, rinse, wash, repeat until the problem goes away.