Pirate Gary’s swashbuckling garb looks more interesting than a chimera or mosaic. Here’s my attempt at diagnosis for any geneticists out there to shoot down if wrong. The first rule of genetic counselling is to look at the family.
Your male white albino parent has two defective pigment production genes and two defective pigment distribution genes (white/leucistic). The wild type female has two normal copies of the pigment production gene and one defective pigment distribution gene. This is why you have 250 normal and 250 leucistic siblings.
This is exactly the sort of cross which exposes in the first generation mutations which would be hidden by being recessive in the wild type.
The single albino can be explained by a new mutation in the formerly normal pigment production copy of that animal resulting in two defective copies and no pigmentation. Many genes mutate at rates of about one in a thousand.
The early pictures of Gary were plausible as a mosaic of albino on a leucistic background and the two mechanisms for how the mosaicism originated have been proposed by others: Both are plausible explanations for Gary on his own but seem unlikely when you consider parents and siblings.
Mechanism 1 - two eggs have become entrapped in the same gel coat and have merged to become a chimera. If this happened between two normal or two leucistic eggs no one would notice, except perhaps if they spotted a hermaphrodite if the eggs were of different sex. If Gary is this sort of chimera he should be a patchwork of normal and leucistic. It seems to me he does not fit this description and you have to start proposing a third improbable event to explain the pure albino bit.
Mechanism 2 is that just like an albino mutant has occurred in one sibling in Gary it occurred in an early cell division, resulting in a white patch on a leucistic individual.
This is plausible and totally satisfying until you see the later pictures. The non albino bits are not leucistic, neither are they normal. Further there is a change in growth associated with the white area.
My favoured opinion is Gary has a new pigment distribution mutation in what should have been the normal copy of the gene and it is exposed by being paired with the leucistic gene from his father.
In summary I am with those that Gary has an abnormal gene in every cell of his body and his markings are like the marble pattern on some fish. He reminds me of a Koi carp. If this opinion is correct it is for the discoverer-you to propose its name but pirate sounds fine to me.
I suggest that in due course you backcross Gary with his parent and then leucistic and normal siblings. Keep records and note what the ordinary ones are as well as the rarities. If none of these crosses produce a pirate fleet you do have a chimera after all but personally I will be disappointed. Gary is obviously viable. He may not be fertile; those small gills worry me a bit that there could be other organ abnormalities. Good luck!