Marine fish rarely get fungus. If the appearance or the disease looks as if the fish has been sprinkled with salt, then you are dealing with ick. The fact that the fish has had this for a long time, suggests that it is ick, and not velvet. Velvet is fast and deadly. Velvet are smaller particles than the salt size ick, and has the appearance of being dusted on the fish [kinda like sandpaper shavings] rather thank sprinkled with salt.
You don't say if you can establish a hospital tank or not, but establishing one would be my adivse.
Also, do you have inverts and live rock, live sand? It is hard to advise you, if we can't get the total picture of what you can offer in terms of treating this fish.
In the meantime, in absense of doing anything while you decide what this problem is, go ahead and start feeding the fish with garlic soaked food. If you at least do this, you will be addressing, to some extent, ick, and, to greater extend, your fishes' immune systems.