I have this very same issue with Gracie Ruth and Yogi. Yogi is very toy possessive. It doesn't matter that the exact same toy is laying right by him, he'll want the one that G has. We've been telling him no and moving him away from G when he goes after her toy. We'll offer him the other toy, which he rarely wants. It's been about 2 months now and he will totally leave her alone when she is playing with Piggy. He's still learning when she has "the bone" or Fish-Fish. But he
is learning. I just have to say, Yogi, no. and he'll usually leave G alone. Of course I'm having to say it a bagillion times some nights.
Yesterday, after I said it, I noticed he was just laying down right in front of her, just staring at her. I'm not sure what fun it could be chewing on a toy with someone just staring at you, but at least he wasn't stealing it from her. If she leaves the toy then it's fair game and he can have it then. She's starting to learn she needs to take it with her to get a drink of water if she still wants it.
Of course there are times that they are playing Keep Away on purpose and I won't stop that. The trick is to know when it's the game and when it's not.
As for the growling, if you question if it's a real growl or play, then I'd say step in and break it up. I can usually tell the difference in the tone with my 2. But if there is ever a question I usually just tell them to knock it off.