Hi, a lot of people had trouble with the green stone. It's unlikely that it disappeared completely, but maybe you've pushed it behind something unknowingly while walking?
In any case, I've changed that specific part of the game in v1.3 to prevent such a thing from happening again. Read more: https://sokpop.itch.io/frogs-adventure/devlog/576568/v13-bug-fixes
Thanks for reporting the issue!