The way his face is described in the book is "His skin was almost the colour of grey, but not quite like and grey Bruno had seen before. He had very large eyes that were the colour of caramel sweets; the whites were very white, and when the boy looked at him all Bruno could see was an enormous pair of sad eyes staring back"
Bruno found his face strange because of how underfed and malnourished he looks - he's described as grey with big eyes which could mean his face is hollow. Bruno grew up in a wealthy family in Berlin so he's never seen people this poor and underfed. His eyes were the colour of 'caramel sweets' which you could link to his hunger.
You could also say that Bruno thought his face was strange because his expression was extremely sad. When Bruno was describing them to his sister he said they 'don't look friendly', so he thought it was strange when one of them wasn't intimidating or mean, but actually desperate and sad
You could also say that Bruno thought his face was strange because he'd never thought about the people individually. When he was looking out of his window he didn't see individuals, just identical people. all of them wearing striped pajamas, all miserable, and all together, so when he saw Shmuel it was a shock because they are real people. You could relate this to the context of the Holocaust because the Nazis convinced people to accept concentration camps because he made people believe entire groups of people were all the same, all evil, all holding back Germany, alienating people from seeing they are actual people.
