A Chameleon Ghoul is the type of ghoul that pretends to be something else.
These ghouls are known to pretend to be suits of armour (CS10). At one time Hermione believed the creature in the Chamber of Secrets might be one. It's possible she read about the Chameleon Ghoul in Gilderoy Lockhart's book Gadding with Ghouls, required reading in Second Year.
A chameleon is a lizard which changes color to match its surroundings, and this ghoul seems to have that same ability.
From the Web
"Chameleon Ghoul" from Harry Potter Wiki