Sometimes the term reverse discrimination is used when discrimination goes in the opposite direction of what is often first supposed.
For example, in the workplace, ageism is generally thought of as discriminating against older workers; however:
It's not just gray-haired employees complaining about discrimination anymore. In this tepid economy, some workers in their 20s and 30s say their age is being unfairly held against them, and new legal developments mean more reverse age discrimination claims may soon be ending up in court. (USA Today, 2003)
A U.S. law firm has an interesting post about reverse discrimination. From a legal perspective, that firm finds it an unfortunate (that is, an unnecessary), because discrimination is discrimination, no matter which direction it goes in. Nevertheless, for better or worse, the term appears to be part of our vernacular.
The basic premise of discrimination statutes is that an employer cannot make employment decisions on account of race, sex, or any other identified characteristic. While this typically refers to making decisions against a member of a protected class, what happens if the employer makes a decision for a member of a protected class and in so doing rejects an equally or better qualified candidate who is not in the class (typically, a white male)? That white, male candidate could make a claim of "reverse discrimination."
Actually, "reverse discrimination" is a poor choice of terminology, because what discrimination laws make unlawful is any sort of discrimination based upon a characteristic like sex or race. Therefore, when a white applicant is denied a job so that a lesser qualified minority applicant can be hired, or when a male employee is denied a promotion so that a lesser qualified female employee can be promoted, it is arguable that there has been "discrimination" because of sex or race, and there is no need to label it "reverse" discrimination. It just happens that the employment discrimination has occurred against a member of the majority class, rather than against a minority class. Still, the term "reverse discrimination" has achieved a popular understanding and continues to be used by many courts.
Reverse discrimination is discrimination against members of a dominant or majority group or in favor of members of a minority or historically disadvantaged group. Groups may be defined in terms of race, gender, ethnicity, or other factors.
So, you could simply call it reverse discrimination. Or, if you need to make the context more specific, reverse sex (or gender) discrimination. Both terms are used.