You know, that does look like bumblefoot, HOWEVER, if it isn't bothering her, I honestly would leave it be. I see lots of people 'operating' for bumblefoot, but if the area is not swollen to the point of having a soft middle in it, like a boil does, I wouldn't open the area up to more bacteria. Now obviously if the chicken/duck is having problems walking, appears in pain, and the area has swollen and has the soft middle, then yes I would treat it by lancing the area, and removing what I could.
Interesting to note, there's been many posts on here about infection in poultry. It seems that they don't have the runny pus from infection that you see with mammals. It forms a hard cheese like substance, from what has been posted by numerous people here. In humans, we would say that the infections has colonized, and isn't a threat to surrounding tissue, organs. Without really knowing, I'm wondering if the bumblefoot that is showing in many flocks, isn't colonized as well, and therefore no major threat to the health of the flock.
Knock on wood, none of mine have shown bumblefoot. Therefore, obviously I haven't had to treat for it. I do know that any time the skin is cut though, you're removing a major defensive line, and opening up the body to many different bacteria. I do NOT know whether this is true in poultry or not, but in humans, staph infections are all the skin. Typically they do no harm. However, if the skin barrier is broken by any means, a cut/scratch/surgery/etc., then the staph can become a problem, and cause infection. If the staph is normally on chickens skin, it would make sense then that cutting that skin opens it up for the staph to take over.
I don't have a bird vet to talk to, but if I did, I would ask if the 'bumble foot' is poultry's way of securing the infection in one spot, aka colonizing it, and therefore should be left alone, unless it's causing other problems.
Sorry for the long post, and I doubt you got much help from it. I'm sorry, as my mind was wandering, and wondering. I hope your duck will be OK.