For those people whom people have met only once or twice in their lives, people often remember the other person by their profession, rather than remembering the person's face. However, it is better to remember the person's entirety- from the face to the figure, from his or her temperament to his or her clothing. This is especially important for clothing with distinctive professional characteristics, such as police uniforms, nurse uniforms, restraint uniforms...and of course, this is including nun uniforms.
The things in a dream are easy to forget after a person wakes up, and when Zhang Zian met the priest in his dream, his attention was paid more to his clothes and occupation. He paid little to no further attention to the priest's face, leaving only a vague impression in his mind.