i'd suggest that it may well be a pied (piebald) mutation. many bird mutations in the pied start with an abnormal patch like this, however, as far as colour goes, i would have expected a yellow patch rather than a white. the known mutations in gtf's are blues and yellows, needing both genes present to produce a white if following normal inheritence as with green birds, eg a pied green budgie has a yellow patch, and you need blue and yellow budgie genes to produce whites.
worth breeding, some nice patterns could appear in subsequent generations.