Artist: Blake McCully of Cross Eyed Air. (Thanks to Barry for the info!)
The mask at the bottom was the one he wore first. Then he got a new shark mask that's very remiscent of Brian Hayward's mask of the early 90s.
Edit: 11/4/11: I've been told by two Sharks' fans that Greiss only seems to be wearing the shark mask during warm-ups right now, and anytime he's in a game, he wears the monster mask. Thanks for the info!