Hi, beginner to fish,
I'm so sorry to hear about your troubles with your fish.
But if that is a fungus, and progressing that quickl...
If you're posting in the Emergencies section, and if you have a test kit, they'd like to know what the water perameters are, especially ammonia and nitrites, how often water changes are conducted, things like that.
It would be good to have these in the initial post, if possible.
Did you lose the last fish to something similar, and is the fish now in the same tank without it being sterilized?
Is he being blown against by the filter and becoming exhausted, as Netty asked?
If he's spent his life in a little cup, he won't have swim muscles - I had one that could barely make the surface and had to be provided with an upside-down floating (clean, never-soaped) icecream pail as a platform, until he could swim properly - and he's fine now.
Stress can cause colour change - and being in a warm tank after an unheated cup can make natural colours appear very rapidly.
Do you think the neons could be picking at him?
Very often the shock of being moved will lower the fish's ability to fight a disease which may have been present but not obvious until later.
There are so many potential issues - more information would be so helpful to the experts such as Netty (I'm NOT one) in identifying the likely problem.
I hope the little guy will be OK, and I'm sure he will be.
Hang in there.