int hot = 20;     // 20x20 square as hotspot

void setup() {
  size(400, 300);
}

void draw() {
}

// we consider a position "hot" if it is close enough to the bottom-left corner
boolean isHot(int x, int y) {
  return (x <= hot && y >= height-hot);
}

void mouseMoved() {
  if (isHot(mouseX, mouseY) && !isHot(pmouseX, pmouseY)) {
    // moved into hotspot from a non-hotspot
    background(random(255), random(255), random(255));
  }
}