To get behind the waterfall in New Pokémon Snap, you must have first progressed through the game until you have acquired the Illumina Orbs and/or the Melody Player from Professor Mirror. You need the Orbs during the day and the Melody at night.
How to move Liepard during the Day
Once Illumina Orbs have been acquired, enter Founja Jungle during the day at whatever Research Level and throw the Illumina Orb from quite a distance away at the Liepard, blocking the alternate route. The Liepard is on top of the boulder just before the waterfall.
When you begin crossing the broken Tree above the Ancient Ruins, start throwing Illumina Orbs, and if your aim is true, the Liepard will jump from the boulder when hit. All you need to do is scan when near the boulder and hold X on the Question Mark that appears to access the alternate route.
How to move Liepard during the Night
To move Liepard during the night, you need the Melody Player. Liepard is fast asleep on the boulder and will not vacate. Press R to play the Melody when you are close, and some Pikipeks should fly away. The Liepard will awaken and give chase. At the same time, you can start throwing fruit at the Pikipeks if the melody does not trigger them to fly away.
Remember to scan the Question Mark above the boulder to access the alternate path behind the waterfall.