set_mouse_sprite_focus man page
set_mouse_sprite_focus — Sets the mouse sprite focus. Allegro game programming library.
void set_mouse_sprite_focus(int x, int y);
The mouse focus is the bit of the pointer that represents the actual mouse position, ie. the (mouse_x, mouse_y) position. By default this is the top left corner of the arrow, but if you are using a different mouse pointer you might need to alter it.
exmouse(3), mouse_sprite(3), set_mouse_sprite(3).