There are certain types of codecs and file types that Mocha does not support, usually native, unprocessed codecs off of cameras and AVIs on Windows. You may need to convert the file to something Mocha supports, such as ProRes or an image sequence. If your screen is black or white for clips but not for images, it is most likely the codec for the movie files that are causing the issue.
View the Mocha documentation to check supported formats.