Wow that's really weird. I'm not sure that I've seen anything like that before.
Can you give some more information about the tank? Particularly the lights, ferts, stocking level, age, CO2 status, and any parameters you have?
There are a few things that look similar to that. Severe nutrient issues might cause it, but with the exception of those few holes off to the side, there's not much sign of deficiency. Additionally, I've never seen nor heard of a change like that.
Lack of light causes something similar, but the part of the stem that gets dark usually turns to mush and breaks off. It will ascend/descend like what you're seeing though, but always turning to much quickly.
It might be a rare plant virus. I'm hesitant to say that simply because I know so little about it and it's so poorly understood. You best bet is probably to physically remove the affected areas and replant what's left.
