There are assumptions and stereotypes about white people. However, such assumptions and stereotypes are examples of Racial Prejudice. Expressions of such assumptions do not constitute racism because they do not have the power/authority behind them (because of where they come from on the hierarchy) to affect widespread beliefs about the group, or to affect the authority, privileges, and access to resources of white people. While expressions of racial prejudice directed at white people may hurt the white person/people individually or personally, and are never be condoned, they do not affect the white person's social/economic/political location and privileges in hierarchy.
Reverse racism is a myth because it tries to ignore the fundamental question of who holds more power/privilege between the individuals/groups involved. The myth of reverse racism assumes that racism occurs on a so-called level playing field (which is another myth). The charge by white people that they can be the targets of racism is a defensive reaction. (See below: How to Identify Racism and Denial/Defensiveness.)
In sum: Racial Prejudice can be directed at white people (i.e. white people can't dance) but is not considered racism because of the systemic relationship of power.