Help, my kitten is acting weird?

So, my kitten is lying next to me on the bed, and he's found a tiny black ball of yarn. He just keeps it in his mouth, basically growling and hissing at it non-stop, and if I try to touch it, or him, he hisses at me and tries to scratch me. He's like 6 or 7 weeks old. What's going on with him?