Hi, You can create an actor as a crosshair and update its position in the world based on where it is with the mouse, (use Greenfoot.getMouseInfo()) and then getX() and getY() on the mouseinfo object after checking it's not null) but hiding the mouse cursor itself is a bit more difficult. Have a look here: http://greenfootgallery.org/scenarios/2828 for a demo if you still want to do it.

I think he don't want to see the mouse-image... For this take a look at this project: http://www.greenfootgallery.org/scenarios/3438 There I published the mouse class, that I'm using for my projects. :) May it helps you, too! :)