Then it could be anything then. Ick can be present in your fish without the white spots like
@phatduckk mentioned, the spots are not the actual ick. Spots can be other things as well.
Marine Ich (Cryptocaryon irritans) - Updated 6-9-2023 What You Need To Know: * Fish parasite which is sometimes managed in a DT by utilizing a UV Sterilizer, Ozone, Diatom Filter, Oxydator, herbal remedies, enhanced nutrition, etc. etc. * Can be treated in a quarantine tank by using...
humble.fish
It's weird that it came and went away though. Maybe stress effected the fishs skin and just made the spots visible. Not a bad Idea to FW dip and check for flukes, this can look like a spot on a fast moving fish.
Probably get more insight at Humblefish if you could snap a photo. Would also assume since you are only observing that you haven't eradicated anything on any of the fish and that you have only weeded out the big problems like Velvet with some luck. Keeping the fish healthy and strong enough to fight off disease will be your plan as whatever disease may exist will be in QT and DT under observation only. Hopefully this is something manageable since the fish recovered right away.