Could it be lymphocystis and not ich? Does it look like cauliflower? Lymphocystis starts as small white growths, eventually they grow and clump together and get the appearance of cauliflower. It is often confused to be ich. Its a virus, with no real cure, the best thing you can do for it provide a healthy enviroment. Good water params, and a healty diet are best, use vitamins and some selcon in the food. It is contagious for other fish, so I would move it back to the QT, and keep a close eye on the fish in your DT. The disease it itself isnt usually fatal( unless growing around the gills), its normaly a secondary infection that kills the fish. In the QT you can treat for bacterial or fungal infections that may arise, but not in your display. It may take a few weeks, but you should see the growths go away.
As for the water quality in your QT, for a new fish the easiest the thing to do use water from a water change on your DT. Theres nothing wrong with this water that you would normally flush down the drain, its perfectly fine to use provided your fish in the DT are healthy.