This watermark is displayed when you are using any trial version of FlashBack.

It is not actually part of the movie so if you open recordings made using a copy of FlashBack that is using a trial licence from a copy of FlashBack that has a full licence, the watermark will not be included.

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.