It is so strange. It's on one of my adults. It's almost like a striping of rust on the body. I'm having a hard time getting a picture, but I will. The shrimp is acting completely normal. It does not act or look sick like a rusting disease. It just started to appear which is odd. I am not sure I like it or not. I will see. It could look really neat but the jury is out. Might end up being a cull but will see.
There is a difference between the rust trait, a racing stripe, and shrimp with sudden abnormal coloration caused by internal bacterial infections (they can act fine for a while before dying). Look into and see what resembles yours.
After researching more, I think this may be the rust trait. Waiting to hear back from where I purchased to see if he gets this trait with his. This does not look like a bacterial infection that I can tell.