Use "trigger behaviour in actor" perhaps, to say to the actors that they must react.
However I don't think there is any neat way of reporting things like neighbor square to north, ... unless you make those as attributes and then assign the correct neighbours for every square you put in the scene. This method would however easily end up in a lot of bugs.
It would be easier (IMO) to do both actor creation and triggering from a scene behaviour in code-mode, though it depends perhaps on the scale of the problem (how big is this grid?). The fast & optimized way would be to have all the actors in a 2D array, and calculate the grid index from the mouse coordinate, e.g. column=(mouse_x-left_of_grid)/pitch_of_grid; Another option is to use the getActorsAtPoint function. In block mode there is to my knowledge no such function, but you could create a region dynamically (surrounding the center actor) and use the "for each actor inside region" as captaincomic mentioned.