Creeping black stuff on Cattleya leaves

[Lady Oscar]

I have a NOID Cattleya that has had kind of a hard life - I noticed yellowing leaves, then repotted (after getting lifesaving advice here!) to find few good roots left. The plant has been hanging on since then, and I can see some new root growth.

However, a couple weeks ago I noticed a sort of textured black stuff on the inside of the leaves of one of the plant's newest growths. This crud has been progressing, and now covers almost all the leaf area on that pseudobulb. The backs of the leaves are still green and unblemished, and none of the other pseudobulbs show signs of the same issue. [Correction - now that I look at the pic up close, the next bulb over seems to have some of it.]

Any idea what this stuff is and how I can get rid of it?

[silken]

I don't know for sure what it is, but I would remove that since it is spreading. Use a sterile blade and cut into green tissue. If you have a fungicide I would treat the plant but at the least dab some cinnamon on the open cut and maybe wipe the remaining plant with some rubbing alcohol. Move it away from other plants. Was water left standing on the leaf at some time? Since it is struggling it is likely more susceptible to infections.