One of the “Slender Man stabbers” — who viciously ambushed and hacked a classmate when she was 12 years old in an effort to please a made-up internet boogeyman — has vanished, and cops are now hunting her. Morgan Geyser, 23, cut off her ankle monitor and escaped from the group home where she living — and she’s currently on the run, according to cops in Madison, Wisconsin. “Her whereabouts are unknown as of Sunday morning,” police said. Geyser, along with her friend Anissa Weier, lured a fellow