If the bird is not scratching..and it appears there are no feathers rubbed from the nares so I would lean more towards the lack of vitamin A as the culprit. If you take it to the vet..or have a look under the microscope you can see the mite within a scraping. Many times the vet will give ivermec as a safe guard but I have brought zebras beaks to top condition with the simple addition of vitamun A and some iodine. Zebras are an australian bird, much like the gouldian and the requirements for these in their diet seem to be very important.
Hope you little zebra gets his red tone back in no time.

Here is an article from a fellow breeder:
Vitamin A deficiency in finches.
by BMustee on Tue May 13, 2008 11:37 pm
.I wanted to post this info for anyone that notices a discoloration in the beaks of their small birds (and maybe their big birds too)
My original breeder female Star Finch came down with a mysterious symptom right after her first breeding season and unfortunately did not make it. After I replaced her and got a second set for genetically non related offspring to sell, another female Star started to get the same color change in her beak. Because there was no info on the web ANYWHERE and the only thing breeders I talked to said it sounded like was scaly face mites I talked to an Avian vet when I took Elmo in for a well-bird visit. He asked to see some pictures and sent me home with some Ivermectin to be sure. I sent him the pics and he was a bit stumped as well, but did not think it was S.F. mites...as was I because non of the other finches had signs of infection. He thought that maybe…just maybe…it could be a vitamin A deficiency. I started giving ALL the finches Necton-S in their drinking water. Necton has the best levels of A on the market and it is a popular brand with breeders. Low and behold, just 2 months later her beak is back to normal. I have sense started giving Necton-E a few times a week as well to boost fertility and I have three nests with eggs..