Umlungu is a slur used to describe Whites in a derogatory way. Zulus called whites this, meaning the white scum that forms in the surf.
The word Umlungu is a racial slur that has been used historically to degrade and dehumanize whites. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to whites. If you are unsure about whether or not it is appropriate to use the word Umlungu, it is best to err on the side of caution and avoid using it in any situation.