Using AMD's Catalyst Beta drivers, if you checked out the contents of /etc/ati/signature it'll show UNSIGNED.
To remove the watermark on your desktop, you'll need to change the contents of this file (as documented in this AskUbuntu thread).
~$ sudo gedit /etc/ati/signature
and replace the contents with
Note: The above should be on a single line. I split it into multiple lines for easy viewing here.
It's simpler if you just made a backup of /etc/ati/signature from a stable version of Catalyst before installing the Beta drivers, then replace the file with the copy you made.