sorry to hear that ur wrasse is showing some sort of parasitic behavior. some wrasse can handle hypo some cant so be easy and drop salinity slowly. i have seen fairy wrasse in hypo and come out just fine some have reported trouble and fatalities.
scratching against pvc is really not a good sign and could infest more if not attended so i am glad u started hypo.
just keep an eye on parameters and also on the fish itself like mentioned before some wrasse can handle hypo some cant. for most part more reefers report success on fairy wrasse with hypo.