This watermark is displayed when you are using any trial version of FlashBack Pro.
It is not actually part of the movie so if you open recordings made using a FlashBack trial in a full version of FlashBack Pro, the watermark will not appear on the recording.
So, if you record a movie using a trial licence and then purchase a full licence, you do not need to re-record your movie to remove the watermark - it will automatically disappear.