Right now I'm staying away from grains as much as I can so I don't eat muesli, but that's for reasons that are not melasma related - I don't see a problem with some muesli now and again, maybe just not every day - make sure there are plenty of seeds in there for zinc and that its unprocessed. I used to make my own.
Quorn, I don't eat simply because to me it's still a very processed food, I prefer to get my protein from beans, lentils, seeds, nuts, eggs, some feta cheese now and again, a little tempeh sometimes. I initially did avoid beans and nuts but I found there was no problem adding them back in after a while, so I eat them freely now. If you wanted to continue to eat Quorn that's fine, it's mainly mycoprotein and as far as I know that wouldn't exacerbate melasma.