From Muppet Wiki

Mo is a stagehand working backstage on Bert & Ernie's Word Play. He and Flo used to be part of a harmonizing trio of singers until the third member with the high voice moved off to Florida. When Prairie Dawn needs a closing act for the show, they incorporate Elmo into their finale, and sing "Need to Read."

The character is made from an orange live-hand Anything Muppet.