Any chance of going back, and taking it, or another like it, into protective custody for rearing to get an adult moth, photographing that, then taking that back for release where you found it?
The black thing is a stonefly nymph (I think). I am hoping Kim or Roger can help ID it. It was trying to attack the caterpillar, coming from below the surface of the water. Unidentified on 12 Aug 2018
The sighting is from August last year but as it was in our creek line, if I see another one, I'll capture it and raise it in our now empty terrarium. Any idea what food they would need?
You could pick a few bits from where you get the beastie, or try plants you get at aquarium shops, Was it in still or flowing water? If it was flowing, you might need to aerate the water.