Black fungus under leaf?

[katsucats]

Alright, so I got this Cyrtorchis arcuata I'm probably going to throw out. It's been infested with some fast spreading leaf spots that I cut out of the other leaves, as you see in the picture. I leave it alone for 2 weeks and come back to this (see pics). There has never been any water splashed on the leaves, and none of the neighboring plants have this symptom. I do have leaf spots here and there that are clearly delineated black circles on other plants, but they don't take over the leaf like this one. As you can see, there is some black mold looking thing that sticks out from the surface under the leaves where it's heavily infested. I've never seen this before.

I just want to know what it might be so I can formulate a strategy in case I see it again in the future.