What material is this in my 1950s home's walls? Is it dangerous?
This looks "Sauerkrautplatte", aka Holzwolle-Leichtbauplatte. It's made from wood shavings and cement, but no asbestos (AFAIK). They used to be common as a form of drywall and insulation (but are not anymore). 


  Wood wool lightweight boards (also called HWL boards ) are building boards that consist of long-fiber wood wool and mineral-based binders . HWL panels are used as plasterable insulation panels both indoors and outdoors. Colloquially, they are often referred to as "sauerkraut plates" or "sauerkohlplatten".