If in doubt, always call your vet or the Animal Poison Line immediately.
Citrus peels, pith, and seeds contain essential oils (limonene, linalool) and psoralens that are toxic to dogs in large amounts. A tiny amount of flesh is not dangerous but the whole fruit, peel, or citrus-scented products should be kept away from dogs.
Usually mild. Monitor and offer water. Contact your vet if symptoms are severe.