Quick answer: This problem is related to screen mirroring on an external monitor. It only happens with MacBook Pro's with a Retina display when screen mirroring is active.
Screen mirroring on MacBook Pro RetinaMany people in music prefer to work on a large screen when creating a song. It is much easier to get a better view on the notes when compared to the sometimes tiny screens of a laptop - screen mirroring to a larger monitor then is the preferred option for many.
This works fine with Liquid Notes on all devices, except for the MacBook Pro with Retina display. Whenever you mirror your screen on this device to an external monitor, it will show with black, pink and white lines:
This erroneous behavior is currently being investigated. If you unplug the connection to your monitor or switch off screen mirroring, Liquid Notes' user interface will be displayed correctly.