Hello
Just a suggestion but you could make the grass out of events and set them to player touch. On the event you would need to set it up like a dice. E.g. Set variable encounter randomly between the number 1-5, then use a conditional branch to check it the variable is equal to one of the numbers e.g. 2 if it is equal to 2 start a battle if not do nothing. That way the play can walk thought the grass and randomly find battles.
Also if you want to make more then one kind of monster able to appear use the dice idea again e.g. if it is set to this number start this battle or if it is set to a different number start a different battle.