Looks like a crab spider. I found that they eat at night too. I saw them carrying away a tiny Pipevine Swallowtail caterpillar and it was gone before I could intervene. Ants, all of them are bad. They farm aphids and will attack almost anything that gets in their way. Slugs, I've seen them eating a pupa that fell to the ground.
Preying Mantis, Crab Spiders, and Assassin bugs wait in the flowers. Some of the butterflies don't even realize what's up until they've been nabbed.http://bugguide.net/index.php?q=search& ... rch=Search
Here's a search on crab spider on the Bug Guide.
They come in all sizes and seem to take the color of the flower/leaf that they are on.