This is Viburnum plicatum f. plicatum 'Popcorn' or more commonly known as Japanese snowball. I love the flowers because of their pretty shape and beautiful color change from lime green to pure white in just ten days.
When the Japanese snowball season is over, the rainy season will start in Japan, which is notorious for humidity but at the same time famous for various colors and shapes of hydrangeas. Japanese snowballs resemble hydrangeas very much, but they belong to completely different genera, i.e., Japanese snowball to the Viburnum genus while hydrangea to the Hydrangea genus.
To see beautiful lime green Japanese snowball flowers, read this post!!