Ghost in the Mirror by R. L. Stine
It's here. It's horrible. And it's in your bedroom! I stood in front of my new mirror and gazed at my reflection. The glass was clean and smooth and perfect. The reflection was incredibly bright, almost brighter and clearer than the room. A chill ran down my back. Fred was staring over my shoulder. "What's wrong?" I asked. "The mirror," he said softly. "I s-saw something moving in there."