In one Boku no Hero Academia Wikia Wikia they mention Shota Aizawa can't erase Mutation-Class Quirks. Which are these Quirks?
Mutation type quirks tend to be irregular body quirks.
Think of Toru Hagakure's transparency quirk .
Mutant (異形 Igyō?)-type Quirks can have a wide variety of features and abilities depending on the individual, but the common similarity seems to be that these Quirks are a part of the user's physicality, causing the user to exhibit irregular bodily features that generally have some sort of purpose to them. As a result, Mutant Quirks are passively active and, at a glance, are the most identifiable. source