I'm sure someone will have a definitive answer, shortly. Meanwhile, I also want to take a guess. I have the XA, but I reviewed the manual online for the XA2 and it's written something awful. My thoughts. . . position the camera so the light turns to green and then cover the lens with your hand. If the light goes out, then it's my guess it's a "good exposure indicator", probably tied to the ASA/ISO setting and internal metering.
If it's not that, then I give up. ;-)