People may have assumed mermaids were white because the mythos on which the Disney film is based is straight out of Denmark, a predominantly white country. Mermaids from elsewhere in the world tend to match the skin colour and general demeanour of the people near where the myth emerges. Mermaids, as featured in the Little Mermaid, are white by design. That's part of the frustration, there's a very functional stories about a kick-ass African mermaid, but we aren't seeing that, we're doing performative things instead.