Generally how it was closed is not the full determining factor. The bigger factor is when it was closed. If it was closed/covered when the water was still warm (above 60) the chlorine you put in it for closing will be depleted in a week or so due to any organic material in the water.. Then the algae has a chance to start growing. In addition when you open is just as important. If you wait and allow the water to warm under the cover then this is more time for algae to grow.
We always recomend - Close late, withteh water the colder the better. Open early, again with very cold water.