Well, there's a simple way to test if the discoloration on plastics is limescale. Get some vinegar, a piece of cloth and scrub. If the stain is gone, it's limescale, if not it's color from the food. We have at home some plastic items perennially stained with tomato sauce, very common.

What pancakes mention is true. The machine can be 10 years old or more and not cleaned that frequently. Try using a cleaning product a couple times and see if the outcome improves.

PS. a stupid joke....could all this be a indirect way your partner tells you to buy new kitchen items? Sometimes the truth is right in front of our eyes, but we nerds start thinking about water carbonate hardness instead of the simplest answer.