Sometimes, especially when a pot is used frequently, you might have to deal with this phenomenon. Limescale deposits are very hard to clean just by washing it.
However, there is something in your kitchen that can do actually miracles:
Rub a piece of lemon on the problematic surface until the limescale deposits are totaly removed. Rinse and it is bright as new!
I Suppose you can also use lemon juice and a sponge.
With lemon, limescale deposits removal is just a piece of cake!
…And mostly important: lemon is a completely natural and healthy cleanser!