As this mask isn't published in the main works of "parade armour" (Robinson, Garbsch, Junkelmann) and does not appear on the Met Museums Collection database it might be a new acquisition or displayed only temporarily, perhaps on loan from a private collection.
Perhaps someone knows more...
To judge from the photo I would say it is a mask of the "Silistra" Type (late 1st to 3rd C. AD)