What should I do about my doubts concerning a man who vehemently slaps his hands on his legs in the mosque, then follows me, and after that I see images of monsters everywhere?

What you are mentioning are mere delusions with no reality, and you are afflicted with a psychological illness. We advise you to recite ruqyah (definition) (Islamic incantations for healing) prescribed by Sharia (definition), and to seek refuge with Allah by reciting Al-Mu'awwidhat (Surahs Al-Falaq and An-Nas), and to cast away these thoughts, strive against them, and seek to get rid of them. Do not harbor ill thoughts about Muslims or accuse anyone without clear proof.