When a larva are fed with dried substrate, a feces can stuck in their anus, and this can be very fatal for their life. If it hasn't been too long, you can try moisten it to break it down with your hands. However, if it has been like this for a while (you can easily figure it out by the appearance of larva being not healthy), then it will be pretty difficult. Your option would be to quickly moisten entire substrate hoping it can naturally be removed. Or your larva won't survive. This can be very fatal, so make sure you always provide enough water!