Mullato is a slur used to describe Mixed races in a derogatory way. Black/White mix, usually. The Spanish "Mullato" translates to "like a mule." Could refer to any mixed breed.
The word Mullato is a racial slur that has been used historically to degrade and dehumanize mixed races. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to mixed races. If you are unsure about whether or not it is appropriate to use the word Mullato, it is best to err on the side of caution and avoid using it in any situation.