The first is that you should check the pH of the tank and change the tank regularly because you have planted floating trees on the water. Secondly, I'm not sure if your light will affect this, but you should listen to the advice above. Love.
I doubt it was the light as others have said, but it is odd that all the degradation in its one spot...
Any output from the filter flipping them over or splashing water on the tops of the leaves? Anything in that particular spot?
6.4 pH is pretty low...are you injecting pressurized co2? How often do you change water and how much do you change out? Normally tap will be alkaline because acidic water will erode the pipelines. So if your tap KH is low like mine, you'll have to change enough water to keep KH replenished. If you can't do that or don't want to, you could add a media bag of crushed coral in the filter to keep KH replenished. But first it's advised to get a KH/GH test so you can closely monitor. A KH of 4 degrees is recommended to keep pH stable.
I would remove any melted leaves and concentrate on the new growth in your tank. Hairgrass is a PITA to plant, its usually a tedious project of planting little groups in a grid pattern where you want it to cover. I think it may be just smothering itself a bit. Can you gently separate it and spread it a bit?