Place a small drop of water on top of the seed shell and leave it. Replace the drop a couple of times over the course of a day and usually the growing plant will have enough strength to finish splitting and shedding the shell. If it's already been like that for a day or two, place a drop of water on it, let it sit a couple of minutes to soak in and then split and remove it manually. I grow almost exclusively from seed and I have to do it so often I feel like a surgeon sometimes.