From the way you describe the pellet it sounds more like a cast than faeces. Owls regurgitate matter like fur and bone that they can't digest in pellets. Break open the pellet with a couple of sticks to see what is in it or, using gloves, put it in a beaker of water and gently break it open in the water. Then pass the water through a coffee filter paper and see what you have left. If it small bones, fur and hair, you may find some insect cases too, you have a cast. If not then it mouldy poo from another creature and you will be real glad you used gloves!!!!