I was going to chime in and say I’ve never heard of this either, but after checking the Wikipedia link I know exactly what this is. Where I am in northwest Iowa, everyone just calls this “overnight oats/oatmeal”. I don’t think I’ve ever heard the word Muesli before.
Ah okay, I just looked at the picture and read the brief description of cold oats left overnight and assumed they’re the same thing. If that’s not the case then I’ve definitely never heard of muesli before today.