Blue tangs are known to be prone to
ICH. They are one of the most sensitive fish to it. Your best bet would be to remove the fish and lower the
salinity of the QT (gradually) or treat with a copper based medication. It is recommended that you remove all fish to the QT, but if I were you I would wait to see what happens with them (as your QT is too small for more than one sizable fish). Your inverts and coral will be fine. There are a few reef safe ich medications available (Kent manufacturers one) but I don't know how well they work. Hope this helps.