int N=30;
float x[] = new float[N];
float y[] = new float[N];

void setup() {
  size(600,400);
  fill(0, 255, 0);
  for (int j=0; j < N; j++) {
    x[j] = random(width);
    y[j] = random(height);
  }
}

void draw() {
  background(204);
  for (int j=0; j < N; j++) {
    ellipse(x[j], y[j], 10, 10);
  }
}

void mousePressed() {
  // let's consider the circles ordered from "front" to "back"
  for (int j=N-1; j >= 0; j--) {
    if (dist(mouseX, mouseY, x[j], y[j]) <= 5) {
      x[j] = random(width);
      y[j] = random(height);
      return;    // this way, we don't move any other circles
    }
  }
}