With Clownfish, one of the first things that comes to mind is Brooklynella aka "Clownfish Disease." And if I'm seeing correctly those white-ish areas near the front of the dorsal fin "could" be early stage Brook. Here's a thread with more info, treatment and pictures of "early" and "later stage" Brook.
Brooklynella hostilis What You Need To Know: * This is most often seen in clownfish, but it can afflict any fish. The fish’s skin will appear to be peeling or sloughing off, oftentimes causing excessive white mucous to form around the affected area(s). * Treatment of choice is a 45 minute...