How to stop my gsd pup biting my older dog?

i have a gsd ***** pup that is 12 weeks old. she is nice around me and does not nip me. i have a 4 year old king charles spaniel also but cie ( my gsd) keeps nipping and biting misty(king charles) especially around the neck. im worried that cie might end up injuring her. is this a sign of aggression or just play? she has not started dog training yet but she wil be starting soon. hope you can help!