When I tap a Potion and swipe left, I see an incubator icon on right bottom. Can I use a potion on an egg?
You cannot use potions on eggs.
The behavior you encounter is a result of how the Pokemon screen was built. When you look at your Pokemon, you can swipe left to view your eggs. This view is reused when you select a potion and are on the screen to select a Pokemon to use the item on. As a result, you can swipe to view eggs on the "potion use" screen, but you cannot use a potion on an egg.
I've heard a theory but I've yet to find anything on the matter yet, but locals in my town are going on about if you manage to get a 10k egg on 7k exactly then you can use a potion in the egg and it's supposed to boost its CP on hatching, working on a 10k atm slowly to see if there's any truth to it but will just says it's heresay for now