Nigrum - Latin, meaning "black".
Therefore, "albus nigrum" is a crude Latinization of the American pejorative phrase white nigger, a person of otherwise caucasian appearance who generally displays behaviors/attitudes of a certain economically-challenged subset of the negroid race.
This term may be used to insult individuals exhibiting these characteristics when the speaker is within close physical proximity to the object of the insult. Generally speaking, individuals who may be labeled "albus nigrum" lack the sophistication and education to correctly determine that they have been insulted, making this term useful in a variety of situations.
See also: wigger, white nigger.
Thurston: It's another unfortunate case of albus nigrum, my good man.