Thank the Good Lord that you are just thinking about it...and it isn't a reality.
Prevention....use a QT and make sure you dip your coral before placing it into the DT..There are worse things than ich.
The truth is, in a reef there is nothing you can do. Keeping the fish very healthy so they are not so susceptible...but if it happens, you will have no choice but to let the parasite run its course...when all fish are dead let the tank sit fishless for 6 to 8 weeks so the parasite dies off.
The junk they sell for ich will make the inverts and coral sick (not kill) and it doesn't work.
I had this happen in my 90g. established reef. I gave up after many tries with different ich medicines. So I tried one last thing...I upped the temps, this makes the fish happier and I fed them shrimp soaked in fresh garlic to hopefully up their immune systems. I got two cleaner shrimp as I heard they pick off parasites...I figured it couldn't hurt.
Well to my amazement...the fish staying healthy, and not being susceptible to the parasite..6 months later no more ich. I even moved and the fish stayed ich free...a very stressful situation and still no ich outbreak.
Some say no way...but I tell you I had ich in my tank for years, every time my fish got a little stressed..ICH outbreak...I killed more fish with ich meds than the ich ever killed, with secondary infections.
All that could have been prevented with a QT.