the vitamins can cause algae to grow, so it is probably that that you are seeing, itl be like little green spots or a green tint.
You can give the vitamin drops on food, you can put it on a little dry food, or on a bit of soft/fresh food.
tofu, scrambled egg, carrot, baby food, cucumber, lots of various veggies, plus they get a little extra moisture, even my pickiest eater likes his veggies and vitamins
i add 2-3 drops a day, or you can give a few more drops if you do it less often, it also is a lot cleaner!
i use a paper cake case for my soft food, so they eat it, if they dont i can easily take it out, and they often shred the empty case into their nest. cheep and cheerful and no more moldy water!