You don't need to do anything. Slugs emit this sticky goo as a defence mechanism to deter predators. Ducks love to eat slugs. They try to get the slime off their bills after by wiping them and shaking their heads and dabbling in water to wash it off (which doesn't work as the slime is sticky). It will pass. They will still eat and enjoy slugs!
This is normal. No need to panic. It's just gross.
If you feel you need to do something, wipe their bills with a cloth or tissue to get the excess off.
I used to collect all the slugs off my plants and feed them to my ducks. They were pest control!