There was some reason Misty went for the puppy, it just wasn't apparent to you
You'll have to become a bit of a detective and watch them to see what is going on.
There are so many reason it could have happened, perhaps Maddie crossed a line she's never crossed before, as puppies get older and more confident they sometimes become a little more cheeky toward the older dog. Perhaps Misty is asserting her dominance. Perhaps Misty isn't feeling good, it's just so hard to tell. Only you can find that out.
The repeated biting is very typical "westie style" in a scuffle. They are determined to get the last word in
. Since she left no marks she was giving Maddie an absolute warning about something. (If she meant to do damage she would have)
I would put them back together when you can supervise them, really watch them to see if you cn tell what Misty's hot buttons are. If a scuffle should break out do no scold either one, just calmly as you can, separate them and make sure both go into the house or into the crates or whatever your method is. That way neither will feel they are being unjustly punished and begin to be
really resentful of the other dog.
Two books I would highly recommend are " Canine Body Language" by Brenda Aloff and "Fight" by Jean Donaldson.
I know it's upsetting for you but it is a somewhat normal part of the settling in period and not totally unexpected, particularly when a playful puppy meets a scrappy terrier. You'll just have to pay attention to them and learn Misty's signs when she's had enough.