The term "grey substance" is a technical term that refers to specific parts of the brain, specifically the cerebral cortex and the basal ganglia. However, if you're looking for synonyms for "grey substance" that are more accessible to the general public, there are a few different options. You could use the terms "grey matter" or "brain tissue" to refer to the same thing. Other related terms might include "neural tissue," "cortical tissue," or simply "brain cells." Ultimately, the most important thing is to use language that your audience will understand, whether that means using technical terms or simpler synonyms.