I believe those are fluke-like, specifically Monogenea, (or some other parasite), which does not seem to spread within the tank. They disappear with time, does not seem to spread onto other fish, (not sure if its the lifecycle or other biological reason they don't spread) It's possible my cleaner shrimps prevented them from spreading. I believe the ones shown are under the skin, which is why you see them as yellow. I may be completely wrong on them being a nematode or flatworm. I didn't pay too much attention to it, so never really investigated them. My fish weren't really affected and I didn't try to remove and examine them. They survived hyposalinity.
Note: There are other species of flukes, which go from fish to fish. That would spread in the aquarium, controlled with (prazipro?) I have also had those in the past, they are almost invisible to the eye, not what you are showing there. These live on the surface of the skin and can be seen flapping in the water, if the current blows backwards. Other species are internal, on the gills, or inside the mouth area, Not visible on the outside. Anyway, a freshwater dip or hypo salinity takes care of these. A cleaner shrimp will clear these.