It is not uncommon to find flies hanging onto the tips of plants, their bodies turning, or already turned, white. The cause is infection by the Entomophthora fungus, which soon kills the insect. The whitening around the wings and the fact that this fly seemed unresponsive makes me think that it was so infected. For other photos of flies fully overtaken by the fungus, please see: http://www.pbase.com/laroseforest/image/119411922 and http://www.pbase.com/laroseforest/image/149410033