void setup()
{
size(400, 400);
colorMode(RGB, 255, 255, 255, width);
background(153, 219, 247);
fill(255, 255, 255, 100);
noStroke();
}

void loop()
{
fill(255, 255, 255, mouseX-width/5);
rect(0, 0, width, height);
}