A mulatto is an individual with mixed black and white heritage. However, some individuals who were designated mulattos may have a slightly more mixed parentage, perhaps including Native American blood.
The 1850 and 1860 censuses only allowed an individual to describe himself as white, black, or mulatto. In 1870, these categories were expanded to include Chinese and Native American as well.
The ethnicity selected for your ancestor in one of the censuses may not be completely reliable, however, because often the census takers did not ask about an individual's ethnic heritage. Instead, they put down an ethnicity based on what the person looked like. Even when asked, light-skinned individuals often tried to hide their non-white ancestry.