float x;
int offset = 10;

void setup() {
  size(240, 120);
  x = width/2;
}

void draw() {
  background(204);
  if (mouseX > x) {
    x += 0.5;
    offset = -10;
  }
  if (mouseX < x) {
    x -= 0.5;
    offset = 10;
  }
  // Draw arrow left or irght depending on "offset" value
  line(x, 0, x, height);
  line(mouseX, mouseY, mouseX + offset, mouseY - 10);
  line(mouseX, mouseY, mouseX + offset, mouseY + 10);
  line(mouseX, mouseY, mouseX + offset*3, mouseY);
}