Could not find a definite answer with a short search. It must be ejecta from the large polar crater Stickney, to the right on the photo. It may not be white at all just looks like it because Phobos as a whole is extremely dark, the darkest object known in the solar system. Almost as dark as soot in your chimney if you fire coal. It is really hard to determine how bright or dark an object is without objective measuring. Who would think that our own moon so bright in the sky is actually dark gray? It still looks bright in reflected sunlight against the black sky background.