My theory is that the man in the iron mask is Jay Garrick's father.
Jay had previously explained that his Flash helmet was his father's, and mentioned that his father wore it when he fought in a war. In "Escape from Earth-2", Barry and Jesse discover that the man in the iron mask was tapping in code used by P.O.W.s (Prisoners Of War).
So I think that Zoom is Jay's long lost twin brother, who was adopted by the Zolomons and named Hunter Zolomon. After discovering of his true parentage, he kidnapped Jay's father and put an iron mask on him to prevent him from speaking. And maybe Jay's father mistook Zoom (Jay's twin) for Jay himself, and attempted to tell Barry that Zoom was Jay under the mask. And when Barry told the man in the iron mask that "Jay Garrick was safe", the man got extremely frustrated, as if he was referring to something else.