float x = 0, y = 0;

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

  noStroke();
}

void draw() {
  background(230, 24);

  // follow the mouse naturally 
  x = lerp(x, mouseX, .05);
  y = lerp(y, mouseY, .05);

  // change the color of the circle as it is near the mouse 
  float d = dist(x, y, mouseX, mouseY);
  if (d < 10) {
    fill(200, 0, 0);
  } else {
    fill(255);
  }

  ellipse(x, y, 30, 30);
}