What are these weird brown spots in plastic tea pitcher?

I used to work at a fast food restaurant and one of the things I was assigned to do was make iced tea. I noticed in the pitchers that there were these hard, brown spots that always came back no matter how much I cleaned it. One of my managers said that it was mold and yelled at my coworkers for it. I’m not sure if it was mold but I never reported it to any of the higher ups. We never cleaned the tea pitchers properly either, just either with a rinse or some soap & water.  Does anyone have a clue as to what it might be?