float x=0;
float y;
boolean active = false;

void setup() {
    size(500,400);
}

void draw() {
  x = (x+4) % width;
  background(0);
  if (mousePressed) {
    if (!active) {
      active = true;
      y = mouseY;
    }
    fill(255,0,0);  // red
    ellipse(x, y, 20, 20);
  } else {
    active = false;
    fill(255);      // white
    ellipse(x, height/2, 20, 20);
  }
}