HELP! Very sick 6 week old duckling and no avian vet nearby

[Three Little Bunnies]

She was roughly six weeks old and passed away about an hour later. Her other siblings seem to be fine. They have access to fresh water to swim in everyday and are enclosed in a wire fence in the daytime, brooder at night still.
I did check the cauliflower that was thrown into their day pen and it did have mold on it. I assume she was the only one who bit into it.

So sorry!