There are lots and lots of threads on here about Ich. I'm guessing that the Yellow Tang is stressed being in a small hex tank. Stress lowers immunity and that's when stuff like Ich grabs them. Almost every story on here where Ich is involved has a fish in too small of a tank. Recommended tank sizes that places like liveaquaria.com use are good to follow. The size of the fish does not seem to matter...the old "it's a really small fish now" logic just doesn't seem to work.
To do this the right way, you need to QT those fish. Treat with copper or Hyposalinity in the QT. other store-bought stuff will not "cure" the fish. Feeding them foods with garlic is supposed to boost immunity, so that can't hurt, it just is not a cure. While fish are in QT, leave your DT fallow for six weeks. That let's the Ich die off due to not having a host.
Those white spots will drop off as it progresses through the life cycle. People think their fish are cured, but the parasite drops into your sand/rock and multiplies. It will come back. Don't let the vanishing spots fool you. It's a good idea to read up on this parasite. There are lots of articles online. It's good to understand what you are up against.