This is almost certainly an opportunistic fungal attack. HC is extremely vulnerable to fungal attack when grown emersed. Infection and damage happens quickly and the fungus is quite aggressive, it will eat your plants and leave sticky piles of digested material behind. If you look closely you'll see thin yellowish spiderweb-like fibers extending from the digested parts, these are the hyphae. Also notice the location of the damaged plants - they form mainly in and around the depressions in the substrate. This is where the substrate is the most wet.
Hyphae:
Fungal infection happens because the conditions are too wet for emersed growth and too dry for submersed growth, the perfect environment for a fungus. So the solution is either to dry out the enclosure more (less misting, lower water level) or submerse the tank and drown out the fungus. The parts that are brown are dead and will not grow back, they have been eaten by the fungus.
Here is a deficiencyfinder article about it with a macro photo:
http://deficiencyfinder.com/?page_id=497
Have a look for the hyphae I mentioned to confirm that the issue is fungus.
Also, providing that the issue you have is fungus, I could add your photos to the deficiency finder article I linked above if you give permission? A few more photos would be helpful if you are up for it (some of the ones you posted have a bit of motion blur).
