float x = 150.0; 
float y = 150.0; 
void setup() {
  size(250, 250);
  background(255);
}
void draw() {
  x += random(-2, 2); 
  y += random(-1, 1); 
  stroke(mouseX,mouseY,100);
  point(x, y);
  if(mousePressed == true) {
    stroke(0);
    point(x, y);
  }
}