If you can (which if it is ick could already be too late) catch it and put it in QT. Even if you are only going to put a fish in observation for a few days prior to putting it in your own tank is much better than not doing anything to see if the new addition has any diseases.
Certain diseases you will know about in observation then you can treat accordingly. I am by no means an expert on this topic. Before I add anything to my main DT, I at least have them in copper for up to a week then observe for another week. Doing this I have been very lucky to not have an ick outbreak or any other disease that I can tell/identify.
I did go through a velvet outbreak and lost a lot of fish. I went fallow until I thought everything was good to add back in fish wise to the tank (for me I did 3 weeks, but had both ozone and UV running the entire time). Fallow period is suggested 72 days.
BTW welcome to the hobby. Try to be patient (key work is "try"). Sure you have read, but nothing happens fast in saltwater keeping besides the bad stuff.