I bought an Ibuypower computer about a year ago, and when we first got it, i had an issue where little gray pixels would flash all over the screen. (in different places for different games) My husband fixed the issue by rolling back the drivers for the graphics card. It was fine until i got Windows 10 and we updated it again, and they came back. They only appear on some gaming surfaces, more commonly 3D rendering stuff. So for example, while playing league of legends, i would see the boxes while playing my champion, but they wouldn't go on the map, heads up display, in the in game shop, or in the client. The only client I've seen the boxes on is the Blizzard client.
It might also be important to note they are not always gray, just most of the time. During some games they seem to be trying to match the color scheme of the background. They also don't inhibit my game play in any way (other than being mildly distracting and make me a little sad to look at), the FPS is the same.