Is there a way to hide the ui in replay mode like in forza 4?
Don’t think so. You have to just let in play by itself until it disappears. So no slow motion 
at least not without extra editing.
Also note that FM6 is always in record mode. So to capture video from playback, let it run for a given duration, then go to Upload Studio and select the record time and, viola, your playback feed is capture and read for upload to One Drive!!! Yeah!!!