It's worth noting that the prop will almost certainly windmill even thought the engine is producing no power. Had this demonstrated to me while doing an FI rating. To stop the engine windmilling it is necessary to raise the nose and bring the airspeed close to the stall. Incidentally, I don't recommend trying it at home. I considered it a rather unwise demo.
A windmilling engine does have more chance of self restarting itself if the ice melts.
Not sure that I agree with Multp. I think a windmilling prop will cause considerably more drag than a stationary one. Open to discussion though.