It happens. You can, 1. Leave it to see if the seedling throws off the testa.
2. Leave it for a day or two and try an remove it by moistening the seed testa by balancing a drop of water on top and softening the seed coat. This can be done several times before trying to carefully pry the testa apart with tweezers, tooth pics, etc. Be careful not to injure the cotyledons held within. My own theory on why this happens is because of a sprouted seed being planted a little to shallow, or planted in a heavily airated soil mix, thus the testa or, (seed coat) does not receive enough moisture to soften enough for the cotyledons to push it off.