Historical settings based on historical humans. This would apply to things like how the Prince looks, or the sailors, or the townsfolk, or any of the other humans.
Ariel is not a human.
Most of the Disney stories are terrible as historical showcases often being a mishmash of elements, more so they have no basis in historical humans, they're deformed and not proportioned like us. That's without considering the fact that for the historical people, mermaids were a phenomenon a lot of people believed in and we have artwork of them in that context, you can look it up if you want, most of what I found showed white skin. That's of course because it reflected our geography, not that mermaids as a concept can't be black. That was what the mermaids were based upon. Heck it's even more a non argument, because historical humans could also have different skin colors and most Disney places are vague general places. Mowgli could be a "white" child, nothing impossible about that. Aladdin and Jasmine could've been "white" as well, nothing impossible about that. They're after all not real people. Unless you base it on an actual historical person, then it's hard to define the skin color of someone and even then there's disputes.
Mermaids aren't real, but neither is Jasmine. There never was an Agrabah, the architecture doesn't make any sense and is more indian, the original fairy tale is from a different setting. It's based on a lot of things from the real world, but it's a mishmash of them and so is the mermaid, it's a mishmash of what we considered real people and the tail of a fish, it's a compound. A lot of people living in the wide area that Aladdin hyper compartmentalized had white skin, especially being of higher social standing. Could you tell me the difference between an Iranian and a Greek person? Why was Aladdin kept more in line with its Disney animated movie and not The Little Mermaid?
What we do know however, which makes your weird argument pointless and it's just using history as a convenience tool ("well you get leeway because you're based on our humans, [but kinda not and historically problematic], but historical perceptions of mythology based on the shape of a real human and a real fish? get outta here), is that Ariel is a human at a point in the story and she is white in the source material and she's white in Disney's adaptation. In the world of the Little Mermaid, Ariel was white, she was based on real historical perceptions of mermaids (the bad or good kind depending on if you're an assman or boobman/faceman). As I said, there was a much better case for turning the Prince black, than Ariel white. The Prince is kept purposely vague in the fairy tale and although it seems like he has white skin, due to other descriptions, it's not something that's blatantly said and gives leeway.
Sure, it's fiction, we can turn anyone the color we want. But a wise person would understand that people generally prefer to keep character somewhat in line with what the original material or historical material. If they don't care, then they shouldn't care whichever way it goes and get used to their characters lacking identity if constant dice rolls are what's going on. As a historian the understanding of the power of identity makes me rather prefer we keep things as they were intended to be, rather than trying to mindlessly appear colorblind. "Any character that's not based on clearly historical humans can be any color, from any adaptation to the next".
No one's dying of this happening, other than the brand of the Little Mermaid, but it's certainly not a good show to suddenly go out of way and not cast according to their own animated movies and more so against the source material. I will at least bookmark this thread and bring it up next time there's any "xwashing" controversy.
