When my celery is at this stage, I am actively harvesting several stalks from each plant every week or so. The celery gets combined with other veggies in mixed packets of frozen vegetables. I never tie up the crowns, just let them keep growing.

I, too, would pull out the troubled soul. I would expect to see root damage due to disease or voles, but you never know.

I don;t know what is wrong with the celery plant, but you really almost can't over water celery. At this stage you should wrap the celery with a square piece of rosin paper very loosely. Measure the height of the plant and cut the paper accordingly. Take the collars off once a week to clean up the plant and extend them as they grow. When it gets near time to harvest take damp cardboard and wrap the plants tightly to blanch them. Your little plants should catch up, there is plenty of time.

I have no clue what causes this, but unless the curling and diminished foliage size is extreme, I would just leave it. My experience is that this condition doesn't seem to hurt overall yield or quality of those plants very much, although I do think there is somewhat of a tendency to be "spongy" at times -- I just use that celery in cooking where it doesn't matter much.

Well, I ended up pulling the celery. It was much smaller than the other 3 I purchased, it the celery was starting to grow twisted. I was paranoid it would spread something to the other celery, which looks great right now! Thanks everyone for the advice.