Update on Natalie--2 vets looked at her, drew a sample from the growth (which wasn't fluid filled), and concluded that it's a proliferated toe pad--which is an extension of the toe pad tissue. Apparently, this is something that happens in Saint Bernards, but not usually ducks. The vet compared it to bunions in humans. So, no infection or fungus. We'll just have to keep an eye on it and make sure she continues to walk around with no issue. Thanks for your input, everyone!