I’d suspect flukes, and if you’ve had ich in the tank before chances are it is still there. It can live on for quite some time as long as fish are present for it to feed on.
I’d remove the fish to QT, with a freshwater dip in transit to check for flukes. Once in QT, if flukes are present they will need treated with praziquantel (2 doses, 5-7 days apart) and copper, ramped up to therapeutic slowly and held there for a least 30 days. After 30 days, copper can be removed with carbon and water changes, and the fish should be observed med free for the remainder of your fallow period.
The display should be kept fallow (fishless) for 76 days to allow the ich time to starve itself out.