If both a Slann Mage Priest and a Temple Guard are in the army, then the Slann must be placed in the unit. The Slann cannot leave this unit- the Temple Guard are spawned to protect the Slann and their temples, and they will not leave their charge for anything. Whilst there is a Slann Mage Priest within the unit, the Temple Guard are stubborn (page 85 of the Warhammer rulebook).
Building an army? Create, validate and share your list with New Recruit, army builder for Warhammer Fantasy Battles (6th definitive edition) and many more games.
Try the builder