When sharing a screen over Zoom in, you might notice a dark gray shade.
This shade is caused by the Compton compositing manager, which you probably enabled in MATE Tweak / Windows / Window manager. Zoom draws a green rectangle around your screen to indicate what is being shared, and Compton draws a "shadow" into this rectangle.
If this happens to you during a meeting, you can quickly "fix" it by stopping Compton from the console:
The shadow will disappear, but you won't have GPU compositing for the rest of your session.
You can tell Compton to not draw shadows around the Zoom window. To do, save the following to
shadow-exclude = [ "name = 'cpt_frame_window'" ];
If you would rather disable all shadows you can alternatively configure:
shadow = false;
Restart your session for the changes to take effect.