Yes that is ich. And it is obviously present in the tank so quarantine is useless (it always is with ich, or velvet, or similar spreadable diseases).
Treatment is easy, and effective when caught early. Raise the tank temperature to 86F (30 C); adjust the heater but also a water change or two with warmer water each time is OK. I always do a major water change when I start treatments, so here I would increase the water temperature several degrees and then rely on the heater to complete the task. Maintain the higher temp for two weeks; one may be sufficient, but ich can be stubborn and just one parasite getting through will reinfest all the tank and fish, so two weeks is best. Then simply lower the heater temp setting and allow the tank to cool back to its regular temperature (which for neons should not be above 76-77F (24-25C). I don't know what other species you have, but most are OK with the higher temperature for this brief a period.
The heat should be sufficient, but if this doesn't seem to work aquarium salt can be used; it is much safer than any so-called ich medication for most all fish.
Ich is now believed to be present in many of our aquariums. Fish are able to resist it if they are in good health and not stressed. It is stress that causes ich to become problematic, always. Newly acquired fish frequently carry it as they are under sometimes severe stress in the store tank, not to mention being chased around with a net and bagged and introduced to a totally new environment. Even if you do not see it, it may be in the aquarium but as I said it is stress that causes the fish to succumb. Flashing is frequently the first sign as ich attacks fish in the gills first; if it is successfully fought off, that is as far as it will go. I have seen this with many new fish in the quarantine tank.
