Two things to try.
1) Take a screenshot of the area and then view the screenshot. Make sure to move the screenshot around to see if the defect is actually captured in the screenshot.
2) Connect an external display.
If you do not see the issue with either of these two tests, then the laptop's Display Assembly needs to be replaced.
Otherwise if you do see it in the moving screenshot or on the external display, then it is most likely a GPU issue or possibly a software issue. Try booting into Safe Mode (screen may flash or flicker due to a very basic video driver being used, but that should not affect the test to see if the video defect still exists. An intermittent issue can be difficult to diagnose. I've seen a few other posts recently with a similar issue.
Hopefully the issue won't get any worse until things get back to normal so that you can have an Apple Store or an Apple Authorized Service Provider examine the laptop.