When units from the army of the Potomac were redeployed to the western theater in autumn 1864 they were welcomed by their western comrades with friendly criticism of their neat, regulation appearance.
What was the nickname the westerners gave the new arrivals?
The only nicknames which referred to the "neat appearence" of the Easterners that I could find in several sources were "paper collars " and "white glove gents".
Potomac Rats, are what they called Hooker's Troops as they were coming up from Bridgeport to the base of Lookout Mountain. This would be October of 63. Doubt if the AOC changed their opinion of them.
Edit - Good try, but I wouldn't think that maintaining a neat, orderly appearance would cause the Eastern troops to be nicknamed "Rats."
I found a book that had several nicknames. Question title is plural, question body is singular, so I am a bit confused on what to respond. I have found paper collars and white glove gents in numerous sources. Undependable holiday Soldiers is also mentioned in several sources.
If the question is looking only for one name, I will go with Paper Collars. You can also find that term here in CWT slang with a similar definition.
Edit - The sources you supplied support only your first answer (you corrected the spelling in a later post). You may thank Wallyfish for indicating that there are sources supporting your other two answers.
Paper collar: Derisive term used by western Federal soldiers to refer to eastern Federal soldiers, who they often thought looked like fancy dandies.
This is the CWT terms definition for Paper Collars that lead me to further search for Paper Collars. I have used this CWT board to find many sland and terms.