First, I want to say I'm no expert, but I'll give my opinion based on my own experiences and limited knowledge base. I agree with
cockroach
and
kno4te
, quarantine if possible but the cat looks pretty rough which means whatever it has, has probably spread to other tankmates.
In the pic there appears to either be free floating particles or perhaps planaria on the glass? Neither of which would be positive sign. I'm not sure due to the quality of the pic.
The fish appears to be covered in as cockroach said growths, The picture makes it hard to tell but it could possibly be ich. The over saturation of light but lack of detail/blur in the pic makes it hard for me to discern. Also, as cockroach mentioned there is an injury on the cats flank. Possibly caused by the cat scratching/flicking/flashing due to the irritation of the growths on it's skin. Or from it trying to hide with more gusto than usual because it's sick causing it to scratch itself by wedging between decor, etc.
As for the sunken eyes, I'm sorry but that's the worst case of sunken eyes I've personally seen. In my experience and from what I've gleamed over the years online it's usually a death sentence. It can have many causes all of which are pretty bad such as secondary infections, dehydration, etc. All of which usually means the primary cause has advanced to a deadly point.
The cat hiding and not moving till you forced it from its hiding spot while typical for many cats is also typical for a very sick fish. They are wild animals and being sick makes them a prime target for predators so they instinctively hide signs of disease first and when they can't any longer hide themselves.
One more thing of special note is that cats tend to be more sensitive to treatments than other fish, even salt. They make up for it by being in my experience hardier than many fish.
