Racism is a blanket term for Ethnic discrimination and a whole host of other forms of discrimination, we just don't use 'Ethnicism' probably because it sounds stupid. Central Americans, or even just plain Mexicans are most certainly an ethnicity, and it becomes racist when you decide they are inferior or discriminate based off that ethnicity, say, by suggesting they should have to build a wall to keep their citizens in because they're undesirable in America. 

Also this is a weird place for a political debate, but it beats complete silence, so w/e.