Black fungus under leaf?

[estación seca]

I'm not familiar with this plant other than from reading. High-humidity plants grown in low-humidity environments look like this when they're struggling. I don't know whether that's what's wrong here, but I suspect low humidity is at least part of the problem. Can you fit it into a large vase, jar or terrarium to raise the humidity? I have a few old aquaria I've bought at yard sales I use for high-humidity plants.