Don't use is_layout when call FM REUSE_ALV_GRID_DISPLAY.
Just set the value that you want to display as traffic light to constant ICON_RED_LIGHT (or green or yellow), which is of char4.
In this way, the header 'Exception' is not shown. Instead, the column name itself is shown.
TYPES: BEGIN OF ty_message,
icon TYPE char4,
message TYPE string,
END OF ty_message.
ls_message-icon= ICON_RED_LIGHT. " red Traffic Light
APPEND ls_message TO lt_message
CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
EXPORTING
i_callback_program = sy-repid
* is_layout = ls_layout
i_callback_user_command = 'USER_COMMAND'
it_fieldcat = lt_fieldcat
TABLES
t_outtab = lt_message
EXCEPTIONS
program_error = 1
OTHERS = 2.