UPDATE: Now 23 of the professor's colleagues have called on the faculty member to resign if the allegations are true. That reflects poorly on them, and suggests they have no regard for contractual and constitutional rights to academic freedom, including the right to engage in racially insensitive extramural speech. Absent a finding that the professor treats students or colleagues in racially discriminatory ways, there is no reason for the faculty member to resign (apologizing might be a good idea though!).
ANOTHER: The Daily Mail (not my favorite source, though they often get the facts out quickly on stories like this) reports that the offending faculty member was Prof. Nancy Shurtz, and gives some context for why she was dressed that way at the Halloween party.
HERE IS A BETTER SOURCE identifying Prof. Shurtz, and includes a statement from her.
STILL ANOTHER: Prof. Shurtz issues a written explanation and apology. She exercised bad judgment. Her 23 colleagues exercised even worse judgment. It's now their turn to apologize!
This doesn’t exactly guarantee meaningful work, let alone jobs, for most mathematicians…