Hello Smiler, It is an informal spoken way to refer to someone that his name is forgotten: " Have you seen that latest movie with Actor X and what's her face , that model ?" Here the speaker can't remember the name of the model and says " what 's her face" meaning "what's her name ,that name I have forgotten"!

her face is prety

It is a colloquial term that is generally used by people who are poorly educated. It is sometimes used when one cannot remember a person's name, but it is considered rude. It is much better to say 'a person, whose name I cannot remember...'

it is used when you cannot recall a person by name.