Milky skin is nothing more than excess mucus production. The best cause of this in long-established "no new fish" collections is pH plunges. When the pH plunges, fish hover at the surface and get milky skins. Their eyes can even turn whitish. There's a way to prevent this.

In a collection of fish with new arrivals but which have a stable pH, then your mind should turn to recent Ammonia burn, or perhaps a parasitism. In either event, salting helps a lot. In fact, it can be considered curative in a lot of cases.

In some rare cases, a salt resistant Trichodina, Costia or Fluke infestation may be the culprit. Use of a microscope can help with the diagnosis of this, a lot. Treat whichever parasite discovered accordingly.