Even without the photo, being "a real %&*)$* to remove" nails it as green spot, no question!
My advice is usually tailored for keeping a tank in top condition and under tight control. Seems that's not your goal, and you want to simply solve this particular problem as easily as possible. But do correct me if I'm wrong.
I need a couple of things first. What's your current water change amount/frequency? Or do you only top off? How old are the bulbs?