I am very strict with this sort of behaviour... no grumbling or pushing out of the way from any of the dogs when I decide to cuddle one. When we first got Flash, Dylan used to get very jalous to the point where he would have a go at him. I dealt with that very swiftly by telling Dylan off and send him away! He soon learnt that if he wanted to continue getting my attention and cuddles, he'd better behave.
These days, he still lifts his head (he is usually asleep on my lap) sometimes when Flash approaches ready to tell him off, but quickly remembers the rules and drops his head again...

I can now give cuddles to all dogs together without the slightest problem and any dog can approach me without fear of getting told off by another.
So in your case, I would just push Masie away (gently) and say "you wait" or "go away" or something to tell her that she is not welcome at that particular moment. I agree with interfering as little as possible, but this is one of two situations where I do interfer and that I am very strict about and won't tolerate any type of aggression (even just push out of the way) at all!
Vera