Yes, Hydraxon’s faceplate is allowed. I was not sure how I felt about that at first. On one hand, I wanted to use it because there are only a very limited number of official LEGO silver mask parts. On the other hand however, I thought it should be banned because it is not a Kanohi. So I made extensive research to see if the canon would allow a Kanohi to be shaped in the likeness of Hydraxon’s face. Turns out it is possible. I have written a whole post about it if you’re interested: Could Hydraxon's face piece be a mask? - #21 by b6i6o6