That green dot is your focus aid. It comes on steady when you are in focus -it's to help you focus manually.
In the old days when cameras were all manual focus, the viewfinder had a "split image" center or a "grain magnifier" in the center to help with focusing. Modern auto-focus cameras don't have that aid, so, unless you have good eyes, you find that it's quite tough to get a good focus manually. Also lenses don't help - focus ring is narrow and the roation is less then 60 deg from end to end rather than 270 degs in the old days.
So, the green dot tells you that you are in focus. In my D7100, there's an option to turn it off - don't know if you have that in the D5200 - look your your menu settings under shooting and focus submenu.