$\mathrm{pH}\text{ of }{10}^{-6}M \mathrm{HCl}\text{ is }$

pH is defined as the negative log of hydrogen ion concentration.

The dissociation coefficient of HCl can be considered to be unity for such low concentration and HCl being a strong electrolyte.

pH = -log(1 x 10^-6) = -(-6) = 6