many plants demonstrate similar markings that amount to nothing apart from mithering the hell out of us. On the otherhand perhaps applying a course of systemic fungicides products much like those for roses that have multti purposes. Atleast the feeling of doing something pratical may help your care approach of your plant. A good tidy up removing dead/ dried tissue, decayed media. Implementing a bit of cross -infection/contamination control may do well also. Are the roots good. Appears to have the appearance of a plant thats has faced the worst of two evils - damp and cold. I have never succeeded with these mule eared oncids.
I would definelty consider some systemic action as soon as poss. Damaged tissue can become infected fungal or bacterial and worth a battle, but they do take hold strong so would avoid risk of water contamination from spills/dribbles/spray/misting on other plants until your more confident.