you may want to quarantine the infected fish. Then observe the other fish in your display to see if they start to show signs of ich too. Quarantine when necessary then do a hypo treatment.
It's not worth doing a hypo in the main tank. I lost quite a few fish doing it and I really don't know why. The more expensive fish lived though.
I invested on a really small QT
. I used to be very stubborn about it but found out that there's a lot more peace of mind and a lot less money wasted buying a QT
I don't really know if corals can carry ich. I did find out that, depending on the stage of that parasite, it could last a few weeks without any host fish. So it may be possible that your invert carried ich even if ich didn't affect your coral.
Sorry about the infection. That sucks. I hate ich. It stresses me out.