The white-ish stuff comes from special feathers on the bird's chest that, rather than molting, grow continuously and disintegrate into a talcum-like powder. The powder is used as a way to absorb fish slime and other schmutz that get's all over the Heron as it catches fish and other prey. You can see tufts of down stuck in this bird's beak. It uses it's beak and toes to distribute and remove (after it has absorbed whatever goo needed cleaning up) the powder.
I have also noticed Heron's dip and briefly swish their beaks in the water right after they swallow a fish. I'm guessing they do this to de-slime their beaks. Like most predators, Herons are fastidious about hygiene.
Tom Bradley © 2015