The shape and location of the bird (in the grass) all suggest a Crested Myna rather than a Common Koel.
All the black crows in Hong Kong have dark coloured bills.
Yes, juvenile Crested Myna appear to have long upper mandibles because the tufts of feathers which cover half the upper mandible on an adult have not yet grown (although this one seems even longer than normal and may be a bit deformed). Look at the adult picture in Birds of Hong Kong and S China and imagine it without the crest.
The white stripe is a piece of grass, as you say, but I think you can just see the white wing mark on the bird just to the right and above that.
