In this sketch, I’m using the atan2() function to determine the angle from each grid square to the mouse.

This angle is then used to rotate a thick short line so that each square is oriented towards the mouse.

Wave the mouse over the sketch to see the effect of this.

Here’s the main loop:

  for (int y = 0; y < nbrRows; ++y) {
    for (int x = 0; x < nbrCols; ++x) {
      float px = leftMargin + x*cellWidth + cellWidth/2;
      float py = topMargin + y*cellHeight + cellHeight/2;
      float angle = atan2(mouseY-py,mouseX-px);
      pushMatrix();
      translate(px, py);
      rotate(angle);
      line(0,0,cellWidth/2,0);
      popMatrix();
    }
  }