It may be a pulled muscle, or it could be a B vitamin deficiency showing up.
For a pulled muscle, rest, Epsom salt compresses, and perhaps a tiny dab of T-Relief or comfrey salve. It takes days and days sometimes for the pull to heal.
Here is something I wrote about B vitamin deficiency treatment.
People often get confused about what to use, so please stick with me here
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You have some options.
I would go with brewer's yeast. Forgive me for using capital letters, but for anyone else who may casually look over this thread, NOT baking yeast, NOT winemaking yeast. Brewer's yeast is a nutritional supplement and it provides niacin and a few other vitamins.
Generally, folks who supplement with brewer’s yeast use a tablespoon per cup of food.
Ducklings need about three times the niacin chicks do. And some ducklings get leg problems or seizures if they don't have enough niacin.
If you cannot find the brewer's yeast, or if you happen to have (here come the capital letters again, please forgive me) PLAIN niacin - NOT no-flush, NOT timed release, then dissolve 150 mg niacin per gallon of water for the duckling’s drinking water.
Another approach is to use B Complex capsules, and use the B3 - niacin - levels as a guide. Again, 150 mg B3 per gallon of drinking water.
Keep the little one on the supplements for about 8 weeks.
Sometimes it only takes a few days to see improvement.
You can also see how it went for Qwackers and Bentley from these threads.
https://www.backyardchickens.com/t/857143/treating-wry-neck-in-crested-duckling-edited-title
https://www.backyardchickens.com/t/761308/peking-duckling-with-crooked-neck-help
