Click the control or press M to activate.
The Magenta / Green control, part of the Edit Color Effects control group, allows you to change the tint of the effects you are editing to be more or less green in color. The changes will be shown in real time in the Preview Viewport.
Move the control to the right (toward the green icon) to make the effects more greenish-colored, or to the left (toward the purple icon) to reduce green.
Note that this control affects all of the effects under edit simultaneously - Primary Spikes, Secondary Spikes, Soft Flare, and Ring Flare.
This control does not appear when editing a grayscale image.