Put sweet potatoes in a pot, cover with water, bring to boil and cook until a knife just pierces them, 15 to 20 minutes. Drain, rinse under cold water and let cool.
In a bowl, whisk vinegar, mustard and honey. Whisking constantly, slowly drizzle in olive oil. Season with salt and pepper.
Peel potatoes, cut into 1-inch chunks and place in a large bowl. Add scallions, celery and apple, pour dressing over and gently toss. Cover and chill for at least 1 hour. Toss gently just before serving.