If you want to turn something on or off based on ambient light then yes, a photo diode, LDR (Light Dependent Resistor) or other photo type sensor would work. I assume the windmill is a small motor driven device? You mention a NC Normally Closed reed switch? That would mean the reed switch opens in the presence of a magnetic field? I won't say the reed switch is the best on and off device for the windmill, there are likely better and easier ways to turn it on and off with a photo sensor. Do you have a schematic or drawing of exactly what you have?
Welcome to the ETO forums!
Ron