This is a normal behavior of a damsel. Just let it be, it's fine. If I'm not mistaken they do this because it's dark, so they can avoid the predators, some sort of protection. I guess, they think, "Uh I play dead so I'm not fresh in the eyes of the predators so they won't eat me."
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I'm not sure, though I know that that's how tangs behave.
I used to own a damsel. This one night I saw it looking like you described, I thought it's dying, it was swiming like crazy, but it's fine. Normally it goes to sleep in its house, in one of the lr. Do you have lr for them to hide at night?
Just a little help on that...
stupid_naso