the function works
The mouseMoved() function is called every time the mouse moves and a mouse button is not pressed.
But you have to have something in the function that you want to happen in this case.

1. // mouseMoved()
2. // Examples    
3. // Move the mouse across the image
4. // to change its value
5. int value = 0;
6. void draw() {
7.   fill(value);
8.   rect(25, 25, 50, 50);
9. }
10. void mouseMoved() {
11.   println("moved..." + value);
12.   value = value + 5;
13.   if (value > 255) {
14.     value = 0;
15.   }
16. }
17. //Description     The mouseMoved() function is called every time the mouse moves and a mouse button is not pressed.

fangmdu, I think you might be looking for the mouseDragged() function, which is called when the mouse is moving & a button is being held down. You can check within mouseDragged() to see which button is being pressed using the mouseButton variable.