When he got his Bankai back from the Quincy who stole it it evolved and began to spread on him, over his eye and part of his face specifically. He's not actually Hollowfied, it just looked vaguely like that.
Just realized something funny - Aizen spent decades, perhaps even centuries trying to create a Hollow/Shinigami hybrid. . .and Urahara manages to pull it off in about a month even though its meant to be a brief side affect of getting a Shinigami's Bankai back.