here's the facts the person playing the guy in the mask is white, not black. and he has what looks to be dirty blonde hair and looks older than barry.
even if it ends up being wally, the guy there showing us is white....from my point of view that would be a stupid move, it's great if your pulling a prank but if your writing for a tv series and making the show somewhat believable you failed miserably by doing something like that...you gotta keep the character the same with/without the mask on.
my guess is its someone related to barry somehow in his bloodline, remember zoom told barry to keep family close, and that the man in the mask would blow his mind if he told him, i'm thinking it could be henry's mothers father, who is the real jay garrick probably played by the actor playing zoom/hunter. I'm probably wrong though.
some say it could be earth 2 henry allen, but unless the writers imply that earth 2 noral allen remarried after henry went missing (zoom took him), that would be the only way it would make sense, to say that biologically earth 2 henry looks different would not explain how earth 2 barry looks the same as earth 1...so really earth 2 nora remarrying is the only way it would make sense for earth 2 henry to be the man in the mask...plus remeber on earth 2 when earth 1 barry spoke to her on the phone and she said her and his dad were together, they were just getting back from somewhere i think.