It is possible that the light level, though not registering as anything to human eyes (which were probably accustomed to the headlamp light at that point as well), could be sufficient to enable some plant growth. This would indicate that you were still in the "entrance zone" of the cave or possibly in the "twilight zone".

Green coloring indicates that the plant is one which is accustomed to receiving sunlight , since that is the best option for plants trying to capture sunlight. Depending on how frequently people with headlamps pass through the area or what infrastructure is in the cave, it is also possible that this plant is an example of "lampenflora", which make use of electric light.

You don't think there was any electric lighting in the cave? I found this:
Plants That Live in Caves | Sciencing here .

"Apr 25, 2017 - Plants can also grow in caves equipped with electric lights. These plants, known as lampenflora, tend to be less vibrant in color and somewhat disfigured. Typically, lampenflora aremosses, ferns and algae.

This link is also interesting . I can't find anything about plants in the Lost Creek Falls cave, but it looks like a beautiful place.

I'm guessing there must be the '0.04 lighting' you mention in the cave, but I agree, it's surprising and amazing.

That is indeed curious. Was the plant that you encountered rooted, or was it just sitting there? Based on your description, I would say the most likely answer is that somebody else brought it in before you and it did not have time to wilt yet. If it was rooted, then this leaves a couple of possibilities:
(1) there wasn't light when you were there, but there is light at a different time of day or some such that allows for photosynthesis;
(2) the shoot was connected via its roots to another shoot elsewhere that was photosynthesizing. (2) is pretty unlikely because most plants can tell if they're in the dark and won't make chlorophyll, instead waiting for sunlight.