Microaggressions can be defined as a subtle or indirect statement or action against members of a marginalized group such as a racial or ethnic minority.
As a target, I would try to give the person that expressed microagressions towards me the benefit of the doubt. Depending on our level of friendship, I would talk to them privately about my feelings and try to explain to them why what they said/did was wrong.
As a witness, I would caution the person that expressed microagressions towards someone else. Depending on the situation, I would call them out on the spot, or I would wait till we are in private to discuss the matter.