I have an idea, but it is an educated guess. Because I see a cyst on the eyeball, I suspect Epistylis. It's a parasite that presents like Ich, but is generally a feeder on bacteria. If a tank has a bacterial bloom, this usually present parasite can get onto fish. It doesn't kill as quickly as Ich, but it has to be dealt with.
Look it up to see if what you see is what the texts and photos online say.
Epistylis loves heat, so if I am right, the course suggested above would make it worse. I reduce temperatures for this to room temps, which a neon won't mind at all. I then water change about 25% daily (well, 4-5 days a week) for a couple of weeks. I have no access to any med that will treat it, but keep an eye out for wounds caused by the parasite as it settles down. I would also make sure I had a regular as clockwork water change regime going of at least 30% every 7-10 days, to dial it down.
I've had it sit on a few individual fish for months, never spreading to tankmates, and never taking off. I think it was a pet shop inheritance in my case, and the water changing held it stable. Back then, I didn't even know this parasite existed, and couldn't attack it as I should have, but the affected harlequins lived for years anyway. If I had slacked on water, I suspect it would have killed them. I wish I had known then about it liking warm water though.
I can only point you in a direction to explore. I can't see the cysts myself, for texture, etc, but a good site will show you the details you need to check out.