sidebar_online_users template
<xen:if is="{xen:helper ismemberof, $user, 4} OR {$user.is_admin}"> <xen:comment> it was this, changed so only members of group #4 show up<xen:if is="{$user.is_moderator} OR {$user.is_admin}"> </xen:comment>
That was the solution I used, only admins and members of group #4 showed up. Our forum moderators are not part of Group #4 (Moderating group), only super mods are.
<Credit goes to a Jake Bunce post, where that post lives *somewhere* else on xf.com>
If you want regular mods to see your moderators forum, you could have a separate group for them to be in - and set that separate group to also see your moderators forum.