Is a young man's imagining that he is explaining knowledge to people or reciting the Qur'an to them in the Grand Mosque of Makkah considered showing off (riyaa'), which might invalidate his deeds?

What you feel after understanding knowledge and memorizing the Quran is not hypocrisy (riya'), but rather a whisper (waswasah) from Satan to sadden you and deter you from seeking knowledge and memorizing the Quran. Do not pay attention to these thoughts, and seek refuge in Allah from Satan, and continue to seek knowledge and memorize the Quran.