Wonder if it's some gunk stuck in the feathers?

If you zoom in it looks like a broken off branch, but hard to say if it’s stuck or the bird is impaled.

Reminds me of that stork that ended up in Germany with an African hunter’s spear/arrow run through it.

It kind of looks like a leaf of a vegetation to me, but it might not be autumn/winter in where you live. It being a leaf might also explain why there is a curve, but I don't think the night heron wouldn't stop and pull it out or something.

I am puzzled by what is on the side of this night heron