Very good to read that the fish has been separated from any others.
It can be a worm on the external. They are not very common to our aquarium fishes (and many of them have a complex means of reproducing themselves such that they can't in our aquariums). Most like Ed has indicated are internal.
Generally, the parasitic group of worms create a hole into the fish. This makes a Formalin bath treatment risky, since Formalin is not recommended for fish with an open wound.
If the 'worm' is really a crustacean of sorts, then a medication containing an Organophosphate (Trichlorfon & Dylox) is the treatment of choice. A product like Clout will usually work, but this is pretty dangerous (to the fish) kind of treatment. Without a clear picture of what it looks like, up close and clear with high resolution, there isn't much more that can be done in the way of diagnostics.