White marks on black grout

[Max1ne]

We've just tiled the bathroom floor with black porcelain tiles and grouted using Unibond (I think) anthracite (very dark grey / black) floor grout. After cleaning off the dry reside using tile cleaner, the grout lines have dried with patches of white which I am struggling to clean off (I've only scrubbed with water up 'til now for fear of making it worse). Any suggestions?
Thanks,
Maxine

 

[Max1ne]

I thought I had. Sorry.

 

[Max1ne]

I was a bit impatient earlier - I hadn't waited for all the grout to dry. ALL of the grout is now an awful pale grey / white colour. I scraped the surface of part of it and revealed a much more satisfying very dark grey / black colour. If I have to, I'll scrape the whole lot but is there an easier way?
I forgot to mention before, we tiled on a wooden floor and mixed a flexible additive to prevent cracking. Can this cause the grey/white colour to appear on the surface?
Thanks,
Maxine