Hi! As this is more of a support question, I've moved it to the support forums where you're more likely to get the proper help.
Try running a permission mask of the moderator permissions (under the Permission tab in your ACP) on this user's username. Permissions in phpBB can be a bit tricky and complicated as they can be a result from both individual and group level permissions.
More information is here:
phpBB3 Permissions