Puppy resource guarding our other dogs food

Hi all,

Just looking for some advice. We recently got a new Australian Shepard puppy and we also have an older (12 years) Sheltie mix. The puppy obviously needs to be fed more often than our older girl, and so at the moment he gets fed at different times. When we feed him on his own (in his own space) he displays no resource guarding behaviour and is completely fine while eating. However, over the past week, when we feed our older girl by herself, he freaks out when he hears me pouring her food, beelines straight to her food bowl and starts eating her food. When I’ve tried to take him away from her bowl, he has nipped at me. She also snaps at him to try get him to stop (no physical contact, just a bark) but he won’t listen. Just looking for some advice as I can’t find anyone else on the internet who has experienced resource guarding in this specific way, and since he only does it while she’s eating I’m not 100% how to go about correcting it (obviously other than making sure he’s not near her at all when she’s eating). Please let me know if I need to be concerned and how we can go about training him to stop this. Thanks!