Saying that your boss was on you "like white on rice" implies two things:

1. your boss unmercifully had hounded you from 9 to 5; and
2. you wouldn't have been able to escape his vise-like grasp even if you had tried (just like you cannot "escape" the whiteness of white rice, no matter how hard you wash it or how long you cook it).

Unfortunately, the exact origins of this phrase are beyond me ken.

Speaking of variations on this theme, I've run across "like flies on a horse" and "like a dog in heat." I have also seen/heard "like flies on ****."