the green box around it is for middle click, and you can change which element is the middle click selected using that (if you actually have a middle click button ...)

since many people, like me, don't have a middle click button, alt+click is the replacement that you can also use. So you can just middle click or alt-click any element on the screen to sample it and make it your left click selected element.

On linux though, alt+click drags the window around and doesn't even send the click to the game ): so I can't test anything right now ... Edit: windows key + alt click :)

So, it looks like you can only change the middle click selected element with ... middle click. And not alt-click. I'm not sure if that's a bug that should be fixed though, most people will just leave the right and middle click selected elements the same all the time.