Screen Sharing Issues

I was helping a colleague prepare for a Visa Project this morning. When he shared his screen I saw a checkered black box representative of the participants he had on his screen and of the zoom controls bar (audio mute video stop, chat etc.)  I am sure it is a setting but we were not able to find it in the time we had. Does anyone have a suggestion or knowledge of what would cause this problem? We can't fix it for today's presentation so he will muddle through, but we need to resolve it before the next presentation.