Marcus is right. This is probably a Joomula issue related to Linux.
If root is not on your Moodle site, then you should be OK. You want to be sure in Moodle that any time a password or profie is changed for admin, you get an email.
I think the issue is that the Joomla interface can pass along authenticated users to Moodle.
If you don't trust your web master, you have the wrong person.