It's a very unique form of Expressionism though that kind of stands in stark contrast to the Expressionist films of the previous decade. I was listening to a bit of the commentary after I watched the film, and one of the dudes on it was saying that by that time in German cinema, they had transitioned from surreal expressionism, and more moved into displaying stark and cold facts through embracing realism instead.
Wikipedia like half-classifies it as Expressionism, but to me, I think this is the best description of Expressionism; "Expressionism is a mode of representation whereby internal feelings and abstract concepts are displayed externally, often at the expense of realism and artistic convention. Expressionist art usually has a surreal or fantastic quality to it, presenting distorted aesthetics through which the true nature of a thing is belied in its external countenance."
Granted, M story wise, very much draws upon the Expressionist movement's use of dark topics and moral questions of evil and it's societal impact, but I do think M is rather special and unique in its use of realism over surrealism. At very least, definitely different from Dr. Cagliari or Nosferatu.