Only if the unit is outside the monk’s areal view. If it’s within the monk’s view the monk will wander to the unit in order to heal (unless other injured unit is close by). Of course you can still use the heal (or right click with monk) on the injured unit to redirect the healing to the unit you want.
Monks will heal a single unit per monk automatically / if you want multiple monks to heal a single unit for faster heals manually select - in groups healing a single unit the first monk heals at the full rate while the rest of the monks heal at half rate.