Are they fish lice or gill flukes? There is a huge difference, and it sounds to me more like a gill fluke.
Fish lice tend to cover other parts of the fish, especially along the base of fins and anal opening, and caudal fin. They are a parasite that feeds on the fish, and needs a living host to survive long term.
With gill flukes, they tend to stay almost entirely in the gills, and will cause irritation and damage to the gills. Gill flukes are highly transmittable in saltwater fish!!! Fish lice can also be highly contageous, and hard to get rid of, though I'll have to do some looking up to find out if they can survive in saltwater.
My suggestion would be to stop feeding your fish anything that doesn't appear to be healthy and/or shows some form of infestation. You could easily infest your entire tank and/or make your fish very sick.
Do they look anything like this?