The concept of race in Europe (especially at that time) is not the same as the concept of race in the US. The Slavs may have white skin, but white skin isn't all that's important. Ethnic heritage creates the basis for discrimination much more often. This was even the case in the US to a degree. Irish and Italian immigrants by this point we're certainly considered white, but they were still discriminated against as unequal to German, Dutch, and English descended Americans. In Europe at this point, there's even a racial divide between Northern and Southern Italians. The cultural characteristics of an ethnicity were (by bigots of course) thought to be passed down by blood to a certain degree. So the races recognized in Europe weren't as simple as "white, black, middle eastern, Asian, etc." There was a lot more nitpicking at traits other than skin color to define a race.

So how did the Nazis define a race? Nazi society was structured using strict racial guidelines, and many problems arose from this because they couldn't always agree on who was German, who was Aryan, and who was even important enough to acknowledge in their racial conversation. The Hitler youth handbook outlined 6 different races native to Germany with the differences between then being hair color, hair type, shoulder shape, and other such things. Those 6 'races' don't even make up all of the Aryans. They had different types and degrees of Aryanhood as well. Mediterranean peoples were considered to be on one of the lowest rungs of the Aryan world, but Aryan nonetheless. Iranians were considered to be Aryans, albeit of a different sort that was not seen as equal to the European Aryans. There were the Aryans of the Scandinavian world, honorary Aryans like the Japanese later on, and the Aryans of Central Europe, i.e. the Germans and Austrians. Even the Nuremberg Laws, after revision months after originally being passed, only referred to 'Jews' and 'citizens of German or related blood.' Who counts as being related to Germans? This question ends up leaving a lot of Afro-Germans in a sort of limbo for a bit, but that's a story for another post. The point is that despite clear-cut racial definitions being essential to the Nazi worldview, their actual definitions, classes, and identifications of races were complex and often contradictory.

So what do Nazis think of Slavs? Well, they see Slavs as being of an entirely separate race, less sophisticated than the so-called 'Aryans', and under the control of Jewish Bolshevism. I'd like to make it clear at this point that when considering German-Slav relations at this time, it's more than racial, but also political. It's not just German vs Slav; it's Nazis vs Communists. Communists, of course being the greatest enemy of the Nazis (in the Nazi worldview, Jews play a large role in Communism around the world so they're to a large degree included in that). WWII in Europe is not just a war of nations, but a war of peoples and races. At least that's how the Nazis, and to a certain extent the Soviets, understood it. So the Nazi attitude towards the Slavs had little to do with their skin color, but much more to do with their heritage and the political structures of their respective countries.

I hope that this helped, and that it wasn't too much of my own ramblings.

Sources :

The Nuremberg Laws

World War One: the Global Revolution, by Lawrence Sondhaus

The Official Handbook for the Schooling of the Hitler Youth

Hitler’s Black Victims: The Historical Experiences of Afro-Germans, European Blacks, Africans, and African Americans in the Nazi Era, by Clarence Lusane