With a loss of pressure and an emergency descent, when at FL100 or below its masks off, which makes communication alot easier and nicer. If its smoke then unless you are 200% sure you have stopped the source AND you have cleared every last bit of smoke from the flight deck I would suggest you leave the mask on. Its never happened to me but I can see no reason to take the mask off if there is even the slightest chance there is still some smoke in the cockpit. Don't forget, it could be poisonous fumes, how will you know? Better to keep the mask on and go land somewhere and check it out I reckon.
I'd open the window. On the ground. Unless something was on fire and it was windy.