Can I mix lawn moss with tree leaves for leaf mould
You can, but as I found out many years ago it's not a good idea.
I raked lots of moss out of my lawn and put it on the compost heap.
About a year later I spread the compost and within days the moss I thought I'd composted was bright green again. It just took a nap whilst in the compost bin and sprang back to life as soon as it was exposed to daylight
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.