When I change to a new world, I want the game to automatically click on an actor by simulating this, not the player clicking on that actor :/ it's hard to explain
Emulating mouse clicks and actually moving the mouse around is possible. However this will not work online, and it is, like Gbasire mentioned, most certainly not the ideal solution. Furthermore, the mouse coordinates are fixed to the screen, not to the scenario, so you first had to get the current window position and size, or move the window. If however you still want to experiment with it, take a look at java.jo.Robot.
Got it! I just wanted to know if I could do that in Greenfoot because in Unity, for exemple, there's a function to do that.
Anyways, I solved the problem by following what you guys suggested. Thanks :)