Teach it the HM move Strength.
You will also need the badge from Celadon City which will allow you to use Strength outside of battle.
You have to find the (hm) strength make a pokemon learn it then when you click on a rock you will be able to push it I dont know where to find strength as I have never played leaf green but I know how to use strength
Moss Rock does not exist in Leafgreen as it was introduced in Diamond, Pearl, and Platinum.
In The Navel Rock
you find it
no
Watch leurois Pokemon leafgreen walk through part.
its navel rock u idiot
in your bum bum
Navel Rock
rock tunnel
ISLAND 1
You get the TM STRENGTH and then give it to a Pokemon.wohoo