As far as I know, the information on that can be found in Cloak and Dagger, one of, if not the very last, 2E products for the Forgotten Realms.
While I've not heard of the Moonstars at all, the way I understood it, Khelben split with the Harpers when they condemned his actions in giving Fzoul Chembryl the fabled Scepter of the Sorcerer Kings in exchange for a pledge that the Zhentarim would stay away from the entire region around Waterdeep for the next ten years, iirc.