It's rotting and I think there is nothing to save now. The plant is small and infection probably got everywhere. Reason is incorrect watering - too much water at once. I recommend misting ionanthas instead of dunking them. After watering they need to get dry fast, it's good to place them at comfortably warm dry spot. About frequency of watering - it depends on local conditions. I usually mist my plants daily in summer and once per 4 days in winter.

It's rot and beyond saving. For air plants, it's best to start with shorter soaks and increase gradually if the plant shows signs of dryness. 4-8h is too long for a tiny plant like that, maybe 10-15min would be a safer start for this size. If you're extra cautious, you can try misting as raxidor suggests. You'll know your plant is sufficiently watered when it looks perky and isn't developing any dry tips. It's okay, start over with a new plant! ;) I've lost 1 air plant to rot too and it was caused by the longer soaks I was trying.