This spider belongs to the Thomisidae or Crab spider family. This is perhaps a Goldenrod crab spider (Misumena vatia) in white, but I am not sure, as it resembles also some Misumenoides and Misomenops species. Anyhow, all comments are welcomed. :)
Crab spiders can be identified by their stronger and often longer front two pairs of legs, which they can hold up, similar to the way a crabs hold their claws. Instead of a making a web, they use these legs to catch a pray (which usually comes by as they sit on a flower). Since some flower spiders can change color -- which may take a few days -- to camouflaged themselves on different flowers, it makes harder to identify them.