User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-1351982-20131016161206/@comment-28861328-20131017154113

It is not "clearly" a Hollow mask. Urahara's pill, as he explained twice, Hollowifies the Bankai "for an instant". Its quick, meant to return the stolen Bankai. Toshiro was said, by Kubo, to have completed his Bankai since the fight with Aizen, but we haven't seen him use it since then, because he got it stolen the instant he activated it; so this is, actually, likely his completed Bankai. The "mask" you're referring to is likely part of Hyorinmaru, just like the rest of his Bankai is. Since when has Hollowfication ever changed a Bankai's appearance? Never did for Ichigo or Kensei.