White bar flashes for half a second at bottom of my sleeping Galaxy screen?

[AC Question]

Hello, this started about two days ago. When I get a notification, my Galaxy phone usually pulses the screen to show me that I've got something to look at. All normal. But, two days ago, I noticed that a bar about a centimeter tall flashes white at the bottom of the screen, above the home buttons. It is only there for about half a second when the screen pulses the screen, displaying the time and the press-to-unlock icon. What is this? Is it something I can fix? I got this phone last year, so it isn't terribly old. Help?