"Manganese (Mn) deficiency is a plant disorder that is often confused with, and occurs with, iron deficiency. Most common in poorly drained soils, also where organic matter levels are high. Manganese may be unavailable to plants where pH is high." From Wikipedia. Perhaps it is time to repot?
I don’t think that is feeding problem or some nutrient deficiency. On the left side the color is pretty dark green. If we have more in information from the owner …
He fertilizers at about 25 ppm N most waterings; plain water once a month, but he only pours about a cup of solution per watering, with no flooding/flushing.
Editorial comment about nutrient deficiencies:
I think most of the published diagnostics don't work well with orchids. They are so slow growing that deficiencies tend to be expressed as slower growth, rather than outward symptoms.