It could be because your surrounding temperature is too warm, causing the oil to separate out. Or it could be that the cream you used was too hot. Try mixing the ganache with a whisk to smooth it. If it does not work, try with an immersion blender and mix the ganache. This should help fix the separation. If it does not,  warm some milk (not cream) and add it in small increments of about 1 teaspoon to the ganache and whisk with a hand whisk until it becomes smooth. Do not add too much milk or your ganache will become too runny.