Please help me figure out what caused this leaf damage

[jocierk]

So I discovered this while I was watering today. I have seen a much less severe version of this on some of my leaves (the damage on top of the leaves, not the white stuff on the bottom) before but never anything this advanced. The whitish stuff on the undersides doesn't scrape off at all. There is also a rusty colored powder in some spots that you can't really see in the pics but you can see it in the discoloration of the water drop (circled). The tops of the top leaves still look totally normal so I think that is how I haven't seen this until now. I honestly don't have a clue what this is and I have done some research and I can't find anything. I did have some spider mites for a little bit due to low humidity (we just moved and I have been working out how to keep up the humidity in the new space) but there were never very many of them and I haven't seen any at all for at least a few weeks now. Is this mite damage? Any help at all is greatly appreciated and please ask if you have any questions that will help diagnose. Thanks!