Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Brother, we really appreciate your forwarding this question to us, and we commend your keenness on getting yourself well-acquainted with the teachings of Islam. May Allah help us all keep firm on the Right Path. Amen.
Generally speaking, the term Muslim minority refers to a smaller group of Muslims living in a majority non-Muslim country. Minorities have rights and duties that are stipulated in national and international laws. They have to respect the law of the country without compromising their faith or identity.
In his response to your question, Sheikh Muhammad Nur Abdullah, President of the ISNA (Islamic Society of North America) and member of the Fiqh Council of North America, states,
"Muslim Minorities" refer to Muslims living in non-Muslim countries where the majority of the population are not Muslims, and Muslims are subject to laws other than Islamic laws. The fiqh of Muslim minorities focuses on educating Muslims on how to strike a balance between their religious obligations and their citizenship duties; how to adapt themselves to the laws that are not in conformity with their faith; how to live Islam in non-Muslim countries; and how to interact with the broader society to achieve the common good for all.
As for the second part of your question, it is a sheer example of racism that is never tolerated in Islam. Islam teaches us to greet whomever we know and whomever we might not know. While greeting others is a recommended act, responding to the greeting is obligatory. If you did your part by initially greeting another person but unfortunately you got no response, there is no blame on you and you should try your best to educate the other person on the civilized teachings of Islam.