So far, there has yet to be any official mention of the mask by LEGO, and every source image comes from the same tumblr page.
However, I'm pretty proficient in Adobe Photoshop and have a pretty keen eye for things that are faked. From what I can tell, this looks entirely legitimate.
Most people that pull off these frauds like to do it for the attention, and make a huge deal out of it. But the tumblr page in question does little to nothing to talk about the mask itself.
So, I'd say this is pretty legit.