The only part that isn't broken down (like Polter Baldi/Baldigeist) is his ruler, which appears to be perfectly fine. Unlike Baldi, he appears to have several holes and scratches on his body. He also appears to have several sharp claws and teeth. Nightmare Baldi, just like his original counterpart, has a green shirt, blue pants, one strand of hair on the top of his head, and an iconic ruler.
Nightmare Baldi appears as a nightmarish version of Baldi from Baldi's Basics in Education and Learning.