It's in the shell gland and it very contagious. I recommend isolating the duck and removing the pool water because enroflaxin and erythromycin and nyastin or fliconzole is needed over at least 6 weeks. Plus, you have to to wipe it up and disinfect for that long. DO not buy ducks unless your...
One of them was being treated for bumblefoot and then the other one started with this a few weeks after. But it started as laying a yolk egg, then three soft shelled, then no eggs at all. I tried aplicillan thinking she had PID. Then 4 weeks later they both have this