Perception: thinking a minority behavior in a minority group is the majority of said group
I think the term you are looking is out-group homogeneity.


  The out-group homogeneity effect is one's perception of out-group
  members as more similar to one another than are in-group members, e.g.
  "they are alike; we are diverse". ... Perceivers tend to have
  impressions about the diversity or variability of group members around
  those central tendencies or typical attributes of those group members.
  Thus, outgroup stereotypicality judgments are overestimated,
  supporting the view that out-group stereotypes are
  overgeneralizations.