Velvet
I shinned a flashlight on one of my cories an saw gold patches around his gills, is it velvet.
I also caught him scraping himself against my sand.
Hello Tet...
The book on "Livebearers" says the "velvet" parasites are very small and do show up as a fine gold-dust on the skin. The first area to show the dust is the gills. Apparently, the parasite uses light as an energy source, so you need to turn off the tank lights and those in the room too.
If you have chemical filtration (carbon), remove it and start an aggressive water change routine, by changing out half the tank water every few days. Vacuuming the gravel where possible is also recommended.
Most fish pathogens don't tolerate aquarium salt, so if it was my tank, I'd add a teaspoon of it to every 5 gallons of the new water when you did the water change.
Also, increasing the temperature a little will help, no more than 82 degrees should be sufficient.
The combination of warmer tank water, a little salt and flushing a lot of pure treated water through your tank every few days for a couple of weeks should help.
I'm a water change fanatic and change out half the water in my tanks every week. This will guarantee chemical stability and that's the most important thing for a healthy aquarium.
Just a couple of thoughts to consider or not.
B