Why are HCl fumes white?
Given that the fumes are scattering droplets, I would say that polar HCl acts as centre of nucleation for water vapour.

This is about the same way by which nitric acid containing small amount of nitrogen dioxide fumes white (in this case first the red-brownish oxide reacts with moisture to form the colorless acid, the latter in turn works as nucleus for water condensation and acidic droplets formation, ie white fume).

A more detailed or perhaps better answer should be under paywall:

Chang, O.-K. Why does concentrated HCl fume? J. Chem. Educ. 1985, 62 (5), 395. DOI: 10.1021/ed062p395.