I can’t say for sure, but my guess is that it was eliminating waste. As you might know, birds and reptiles don’t urinate and defecate in quite the same way that mammals do. Mammals like us get rid of excess nitrogen (from breaking down proteins) as urea. Of course, we urinate that out. It takes a significant amount of water to flush it out of our bodies. Obviously, defecation (pooping) to eliminate digestive wastes is completely separate.

Birds and reptiles are better at conserving water. They produce uric acid from their excess nitrogen, which they can get rid of as a paste in very little water. Not only does this help when water is scarce, it means that birds have to carry less water around. Water is heavy and things that fly, whether they’re jets or birds, need to reduce weight. Digestive waste is mixed in, and all of it is eliminated at once in droppings. They only have one opening for reproduction and elimination. It’s called the cloaca. This also reduces weight.

Owls save even more weight by producing pellets of fur, bones, teeth, feathers, or insect exoskeletons. They vomit these back up before they get far down the digestive tract. There’s no point in having a gut full of things you can’t digest.

Can you think of any reasons that the egret might defecate into the water, rather than when it was standing up?

Everyone poops.

Birds excrete nitrogenous waste (formed through the breakdown of protein) in the form of uric acid, rather than as urine the way mammals do. This comes out the same opening, and generally at the same time as, digestive waste, which explains how bird droppings are produced. I admire the poetic language used to record your observations!

My guess is that it was bird excrement, but can't be certain from your description. Bird excrement is oily and crystalline, since birds extract the water from it before excreting it.

It was probably egret poop! Here's a nice youtube video: watch here

Here are some experiences I found online when searching for 'milky' or 'scum,', but I think only the 'poop' answer is correct - especially since it came after the egret ate the fish!

1. I have seen Gulls defecate over ponds and this leaves a milky white film on the surface.
2. Yesterday, I opened my back door to go out to feed the fish, when I saw a heron flying off. I thought we had been lucky to avoid it so far, so it was only a matter of time. I think it took 5 fish. When I looked at the pond, it was covered in a film with a white milky appearance, it was a bit like an oil slick on the surface of the water where the heron had been. I am intrigued to know what this is. I cannot find any forum at all that discusses this. Someone had told me that herons produce a substance in the water to attract the fish to come to it, (like a pheromone?) but I have not heard of this before and cannot find anything related to this on the web.
3. For those that havent yet had the pleasure this is a calling card left by our feathered friends here in the UK, I believe US Herons leave a similar scum.
4. I don't know the exact origin of the scum but every time I have seen the scum I have either seen a heron or suspected one has been visiting.