Please help identify these "rusty" leaves on my Jade (crassula ovata)

So I've had these three jade plants for about a month. Got them from a local mother who told me they'd been her daughter's science experiment for school. I planted all three in 50:50 pearlite and local alluvial sand. I am letting the soil dry fully before watering them, tho they DO get "overmist" from when I mist my juniper. Also, the pearlite is MiracleGro brand, and is pre-fertilized 20:10:10 ... not thrilled with that, but it's all that was available to me.
I'm new to bonsai, but not to plants in general.
I've not found anything in my searches that matches what's going on with these leaves, on JUST one of the plants. I only noticed this a few days ago.
Is it disease? Perhaps tiny tiny bugs that I can't see? If either is the case, I should probably quarantine it in a new pot, yes?
If neither, I'm guessing I should trim these leaves off, yes?
Anyhow, here's the image of the problem.

Thanks!

Hmmm ..... nitrogen burn maybe?
Testing that theory by setting a piece of the pearlite on one of the leaves.

further research leads me to believe the most likely cause is sunburn. The nitrogen burn theory occurred to me as I lay in bed trying to sleep last night. I'll leave the pearlite on a leaf for now, to finish testing that theory. The pearlite is, as I said, high nitrogen (20:10:10).

Solved. Conclusively sunburn.
Happened to one leaf on another plant today, which is exceedingly hot and humid, and the sun is like a laser beam out of the sky. It happened in under 6 hrs.
Pulled ALL plants in under the shade. And lightly watered them.