what are the white cotton-like things flying around in Hong Kong? What kind of tree do they come from?

It is Bombax malabaricum has cotton like husks were used a substitute of cotton.These attach withseeds and flow with air for dispersal.This plant is very common near Hong Kong.

I dont know what you have seen, but the possibilities are:

1) it may be cotton from cotton plant

--- The cotton plant belongs to the genus Gossypium of the family Malvaceae (mallow family). It is generally a shrubby plant having broad three-lobed leaves and seeds in capsules, or bolls; each seed is surrounded with downy fiber, white or creamy in color and easily spun. The fibers flatten and twist naturally as they dry.

2)It may be some plants seed

-- visit this site for more

http://waynesword.palomar.edu/plfeb99.htm