By 1930, why did some americans want mexican to no longer be " white"

Because Politicians were afraid that Chaos would broke up in united states due to the Great Depression.
Because of this, they use the Mexicans as the scapegoat and told American citizens that they're the cause of the Depression. This lead to some Americans demanded Mexican people to no longer be considered as a part of their society. 

More context .

Ghana, first of the great medieval trading empires of western africa (fl. 7th–13th century). it was situated between the sahara and the headwaters of the sénégal and niger rivers, in an area that now comprises southeastern mauritania and part of mali.