I’ve seen a lot of duplicate threads asking about this.
The white square is the “flash indicator” which arrived with the new Nvidia Reflex added into Overwatch in the latest patch.
It can be disabled in-game by:
Options > Video > Nvidia Reflex > Flash Indicator > Disabled/Off
PS to Blizzard: This square should be turned off by default!
This is a duplicate of the thread made in #bug-report to reach people in this category.
