Silver-haired humanoid?
Well, in the original web novel Rimuru's slime form is silver and the human form is described as having silver hair. But in the light novel and manga adaptations they changed it to blue. The anime's art and story are more closely based on the manga than the webnovel, so I'd guess that's why he appears as having blue hair in the anime too.

In the scene you're referring to, Milim does actually call him silver-haired and it's not a bad translation. Maybe blue just looks better on screen, I don't know why they do this. Although, there are some other anime too where the hair color looks different from what it's supposed to be, I don't pay it much heed.