White horizontal lines are retrace lines. There are a few different reasons you can get retrace lines but if you are saying the screen is predominantly green and has white retrace lines the component that comes to mind is the PRA-B board for Green, or one of the three transistors around it, most likely 83G if it is a transistor. I suspect the PRA-B board though, the SMD video amplifier IC on those boards tend to run hot and after 20 years or so can short open which causes the color to drop low. The retrace lines are caused by the auto bias circuit reacting when it notices the color is missing, it keeps turning it up which overdrives the image causing retrace lines, just like when a flyback is turned up too high.
Chris