It's called a "hypo" sunfish,
The same regular fish but with a co-dom gene called Hypomelanistic, it's a rare find. To help you understand what a Hypomelanistic gene is just think of any albino animal and that will help you understand. It other terms, a genetic f*** up. It you breed it, half the babies will carry the gene and come out hypos.
I breed snakes and do a lot of work with gene pull and selective breeding.
I may be wrong on this but I'm 99% positive that's what's going on with your fish.