float x,y;
float eps = 2;
float r = 20;

void setup() {
  size(500,500);
  background(255);
  x = width/2;
  y = height/2;
  frameRate(240);
  noStroke();
}

void draw() {
  fill(180);
  ellipse(x,y,r,r);  // gray out old spot
  x += random(-1*eps,eps);
  y += random(-1*eps,eps);
  fill(255,0,0);       // draw new in red
  ellipse(x,y,r,r);
}