User blog comment:Dbxkilla/Hollowfied bankai/@comment-7606564-20131021192645/@comment-26420452-20131022213441

The "mask" seems to be made out of ice, not of the typical material Hollow masks are made of. Plus, Suì-Fēng's and Hitsugaya's battles are happening more or less instantaneously, and she didn't show any sign of Hollowfication. Most probably, the change in appearance is due to Daiguren Hyōrinmaru now being a complete Bankai.