A Mufti is a 'Alim who, after his 'Alim course/darse nizami, went on studying several more years specializing in Iftaa (sciences related to giving fatawa). So, every Mufti is a 'Alim (scholar), but not all 'Ulama' are also Mufti (only a portion of them who added Iftaa studies to their "curriculum").