It doesn't appear to have a disease.
Are the other rummynose tetras eating and swimming in a group?
It might just need time to settle in and recover from being moved to a new tank with potentially different water. Fish are like people, some people don't suffer from jet lag, others do. Some fish are tougher than others and don't stress as much when moved.
Give it a couple of days and see how it goes. It's not going to starve in a few days and if it still isn't acting normally, maybe do a 1 minute video and upload it to YouTube, then copy & paste the link here.
Maybe find out what the pH,
GH and
KH are in your tank and the supplier's tank. If ther eis a lot of difference, that could cause this, especially if the fish had only recently arrived at the suppliers.