Clownfish can live with Ich just fine. In time, their immune system will become familiar enough with the parasite and release an antiparasitic protein into the mucous coat to fight off the theronts/trophonts. This is similar to what rabbitfish do:
https://www.ncbi.nlm.nih.gov/pubmed/20117218
However, if this turns out to be brook that is a completely different story:
https://www.reef2reef.com/threads/brooklynella.247938/
P.S. This "antiparasitic protein" strategy doesn't work too well for fish with
thin mucous layers: Tangs, angels, butterflyfish, grammas, etc. So, anyone wishing to employ Ich management is better off sticking with clownfish, wrasses, anthias, cardinalfish and others with thick slime coats. Utilizing a UV sterilizer, diatom filter, ozone or an oxydator will help control parasite populations in the water. Feeding high quality seafood with vitamins/probiotics added in will help boost your fishes' immune systems to deal with whatever parasites survive and latch on.