Black marks (not spots) on underside of umbrella plant leaves

[Faffifan]

Good day,

I purchased a umbrella plant from a local nursery here in Ontario a few weeks ago.
The plant is growing on a hunk of volcanic stone and you water it buy filling the bottom basin. This thing drinks up the cup of water in 2 days !
The white mold that grows on the rock is apparently not dangerous according to the info card. There is no leaf drop and the leaves arent wilting. It sits in a south facing window that gets direct sunlight most of the day.

I noticed there are black marks on the underside of the leaves.
The marks are slightly indented on the bottom side.
I cannot see the the black from the top of the leaves. But there are slightly raised lumps on the top of the leaves.
Almost all of the leaves have a little of this.

Also 1 cluster of leaves has a yellowing transparency to it.

I noticed a spider mite infection about a week ago and I have treated them with isopropyl alchohol spray and they are almost cleared up. Are the black marks scarring from the mites?

I cannot compare this to any fungal infection on the internet. So I'm scared to use a fungicide as umbrella plant is apparently sensitive to them.

Thank you in advance for any help you can give !