DATE_ADD(now(), INTERVAL 6 MONTH is SQL code, not PHP.
try using the date and strtotime functions instead:
<?php if(date("Y-m-d", strtotime($row_WADAclients['StagingDate'])) > date("Y-m-d", strtotime("+6 months"))) { // Show if column... ?>
<img src="red.png"/>
<?php } // Show if column... ?>