The phrase "undifferentiated mass" means a large group or collection of things that are all similar and do not have any distinct or specific characteristics. Full definition
From this apparently undifferentiated mass of cells a stalk grows at the top of which a fruiting body is formed that develops spores. (religion-online.org)
This was the situation with my own dear Australian Shepherd, Alfie, who had a huge undifferentiated mass rapidly overtake his liver. (homevet.com)
Stereotyping Trump's supporters as an undifferentiated mass of mouth - breathing racists with gun racks and camouflage pajamas isn't any better than Bannon or Trump portraying the media as a hotbed of Eastern, latte - sipping liberals. (fortune.com)