While it is shown Ursula has not only the power to turn a Mer-person human, but erase their memories, even specific memories, there are only moments here and there that make this the slightest possibility. However, when Triton is close enough to see Queen Selina, he doesn’t react so, unless Ursula put him under a spell too, as well as the rest of the Mer-People, she likely isn’t Ariel’s mom.