This problem is due to a default font that the graphical user interface uses not being installed on your system. The solution is to use a '-font' argument when starting, eg:
mercury -font -adobe-courier-bold-r-normal-*-12-120-75-75-m-70-iso8859-4
However, that font may not be on your system. All the fonts on your system can be listed using the command 'xlsfonts', or you can use 'xfontsel' to build up the font string piece by piece, to find something pleasing to your eye. Ultimately, the font string may be added to the end of the last line of the run_application.sh script in the program's bin subdirectory to make the change permanent.