Welcome to BYC,
@Ripecelery!
One of my pekin hens has recently (this month) started having a very small amount of foam some days, I think it's just this time of year that's causing it. I don't really think it's anything but I'm still being vigilant.
Some of my ducks in the past have had some foam or slight discharge around their eyes (it really only seems to be one duck that has it at a time) around this time of year I think it may just be like allergies or something although with any kind of odd or abnormal behaviour or discharge and so on, it's good to make note of it and check to make sure it's nothing serious.
Is there any other things that are off about your duck?
How often do you clean their water bows/containers pond things?
From the picture it doesn't look like his eye is irritated (red or inflamed .etc)
Do both his eyes have discharge and how much?
Does he act like his eye is bothering him?
What kind of bedding do you use?
How long has this been going on and has it happened before?
I don't really know much about most duck and chicken illnesses and the like so please take anything I say with a grain of salt, as I'm not very experienced.
I hope it's nothing serious your duck has going on, good luck.
@New duck mommy 2021
(Edited to fix grammar)