It isnt a race but still falls under racism. Racism is much broader than you think. The UDHR recognizes that if people are to be treated with dignity, they require economic rights, social rights including education, and the rights to cultural and political participation and civil liberty. It further states that everyone is entitled to these rights "without distinction of any kind, such as race, colour, sex, language, religion, political or other opinion, national or social origin, property, birth or other status." This is from UN declaration on human rights. However, you are not wrong. It is true that Muslims are not a race. The word “Muslim” itself connects to followers of Islam, a world religion — not a “race.” Muslims are a diverse religious grouping, and, in fact, one of the most heterogeneous populations in the world. In theory, the ummah - or global Muslim community - is made up of many “races.” Moreover, not all Muslims are simply born “Muslim,” like people are born White or Black. Muslim identity is something that one can acquire through conversion. In this sense, Muslim identity is nurtured and not natured. People like to classify this a cultural racism because one group is expressing how superior their culture is over the other. I'm not sure why I went this in debth but I tried to show why it is taken as racism when Islam is not one race.