The skirt could have gotten spooked and charged blindly into the moss. Or it may have been after another fish. Large skirt tetras can get a little nasty, or at least that has been my experience keeping them years ago.
I've always had rotten luck with neons, and I've had people tell me that they are "very fragile". My recent experience with them has been that they are usually the first fish picked on by tankmates, often chased, harassed, and nipped to exhaustion and death. I have a heavily planted/decorated community tank, with lots of hiding places, so I don't know why it's only the neons that are picked on.