Moderators are allowed to see the IP address not just when the PM is sent as an Alert, they see them also in any regular message (by the way, reporting system is also based on the core message sending, there is nothing special). Moderators also see regular IP addresses in any message on forum.
It can't be changed on the optional level. Assign moderators to trusted people only, that's the best solution of this task.
If you want to hide IP addresses in Private Messages for moderators, open addon_pmail.php
for editing, search for `$viewIP` and remove moderators condition like here:
if( ($user_id==1 or checkModerator($mods,$user_id)) and isset($pmIp) and $user_id!=$cc).....
-- remove all parts in bold.