Depending on the context, you could consider hustler. This is the exact and precise word to use if someone is playing pool for money: they pretend to barely know the game, even lose a few small bets to you, then make a big bet and win dramatically. However, the word hustler has some other meanings, so if you try to apply to some context other than playing pool for money, it might be misinterpreted.
If the person’s reason for hiding their wisdom and experience is not to con or hustle you, then there is probably not a good single word for it. False, artificial, feigned or fake1 innocence, inexperience, or naïveté are probably the phrases you could use.
- Or faux, if saying it in French appeals to you.