By default, the menu bar appears on all displays in macOS. You need to change this default to prevent the menu bar from appearing on your video projector output when using Isadora.
1) Go to the System Preferences and choose "Mission Control"
2) Uncheck the box next to "Displays Have Separate Spaces"
3) Close the Preferences
4) Choose "Log Out..." from the Apple Menu
5) Log back in as you would normally.
This article has more information about optimizing up your computer for Isadora.