Muesli is a cold breakfast mix of grains (very often oats), nuts, and dried fruit. People usually pour milk or yoghurt on it and eat it from a bowl. The food and the name come from the German-speaking Alps; the name is a small, friendly form of a word for a mashed dish. It is not hot porridge, and it is not the same as granola, which is typically baked and crunchy — though actually shops mix the names, and a bag of「穀片」may be closer to one or the other. In English it stays a food word; you do not often use it as a joke or a metaphor. It is an ordinary supermarket word, a little European in flavour, not slang.