Update for anyone still willing to help:
I've stopped getting the error for the class now but still no moving circle. Heres what I did to get rid of my error. My MouseMove code looks just the same but instead I've added repaint() Please help!
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
g.setColor (Color.red);
playerCounter = new Ellipse2D.Double(30,30,30,30);
setVisible(true);
if (playerCounter != null){
g2.draw(playerCounter);
}
}