What is this brown stuff in these milk cartons?

I have found several 1/2 gallon cartons of milk manufactured by Trader Joe’s have what looks like a brown slime at the bottom inside the milk cartons. Below, I show two example cartons of milk, one in each column. Each column includes a manufacturer date and product stamp (both are the same) photo. Each column also shows a closeup of the brown slime using a Teslong NTS500 Endoscope.