I've seen a few similar posts about this in the past. Most of the replies said it was just a mutation from other color shrimp in the genetic line, although someone said they had this appear after a failed molt and it disappeared after the shrimp molted again. Not 100% sure but I'd guess it's one of those two options. I would say if the shrimp are breeding that is a good sign, unless you start to see dead shrimp I don't think it's anything to worry about.

 

Without better photos, it's difficult, if not impossible, to say.

Though, it could be a typical coloration/genetics thing that happens with Neos.

If it's brown spot/rust disease/bacterial, you'll begin to see pitting on the exterior of the shrimp and they'll start dying. You can do some searches to find photos of that in order to compare to your shrimp. It's generally something you can treat. But if you don't see pitting, shrimp aren't dying and it's not showing up on other shrimp? No big deal.

How specifically did you cycle your tank?

What are your specific water parameters from a liquid test kit? Ammonia, nitrite, nitrate, pH, kH, gH? Temperature? Do you dose fertilizer? How much water do you change and with what frequency?