The Monoa Mask is unsellable so I echo you try typing /isearch Monoa as the only way you can have actually lost it completely is to actually discard it and had to check the "Discard unique/untradable item" box and then hit Yes.

It could also be in your armory chest and not your bag. The default is to store all newly obtained items in the Armoury Chest which is separate from your inventory. See if it is in there.

If it is actually gone double check the Calamity Salvager, look under "Purchase Quest Rewards" -> "Purchase Quest Rewards I" and you should see it on the very top of the list.
Only reason I can think that it wouldn't be there is perhaps it is only available after you complete the quest.

Also, if you are new you probably only are seeing 1/4 of your inventory at a time so check all the tabs or see it in one pane by going to character config -> item settings (looks like a bag) and on the top change the 'Inventory Interface' to 'Open All' or whichever looks good to you. You can also turn the adding all new gear to armory chest off from there.