How to turn the mosaic effect on and off within a clip

Hello everyone,

I have to cut a lot of MS Teams recordings at the moment. For data protection reasons, the right edge of the screen, i.e. the displayed profile pictures of the meeting participants, have to be pixelated. To do this, I create a mask using the Mosaic effect, which also works (see screenshot).
So far, so good. However, during the video, the screen is split and the column view changes again: the speaker's video becomes visible and of course should not be pixelated.

How can I deactivate the 1st mask here and insert a 2nd mask, how can I adjust the shape again? I've already tried to set keyframes, but somehow I'm not making any progress... Thanks in advance for your help!