Most of these people have obviously not read the books.
If they had, they would know there is no racial component to slavery as practiced on Essos. It is based on slavery as it existed in the ancient world. The Romans and Greek were just as willing to enslave other Greeks and Romans as they were Celts, Goths, Germans, and Africans. It's on the page.
However, when you are filming scenes in Morocco, and you put out a call for extras, it's Moroccans who show up. Most of them are darker skinned than our European actors (though there is actually a lot of different races and ethnic groups represented in the country, including Arabs, Berbers, Africans, French, etc). It is not so different from shooting a scene in Belfast and putting out a call for extras, whereupon a lot of Irish show up.
We fly our actors from country to country and continent to continent, at considerable expense, but that's not a practical consideration when dealing with extras. So in any big crowd scene, the prevailing skin color is always going to echo that of whatever the location is that you're shooting in.
But just for the record, yes, Dany is white, just as she has been from the beginning, and she may or may not be a savior (the last scene in "Mhysa" is not the end of her journey by any means), but she frees slaves of all colors, races, creeds, and nationalities.