こんにちは (Konnichiwa) FrancescoF,
Thank you for your question!
In Japanese, you can ask casual questions by raising the intonation at the end of the sentence, instead of using か (ka).
You can read all about it in this lesson:
In this case, あなた は？ (Anata wa?) “You?" is an abbreviated and more casual version of あなた は げんき です か？ (Anata wa ogenki desu ka?) “How are you?”
Since it's obvious from the context that the speaker is asking how the listener is, we can drop the おげんき です か？ (Ogenki desu ka?) and just leave it at あなた は？ (Anata wa?)
You can think of this like the difference between “You?” and “How are you?" in English.
I hope this helped.
Please let me know if you have any other questions.
べんきょう を がんばって ください！ (Benkyō o ganbatte kudasai!)