From the novella, we cannot be sure his name is Arthur. I used this novella for my WL1, and I think the name Arthur originated when it was made into a film.
Hope that helped.
From my understanding, sweettangerine is right... nowhere in the actual book does it say his name is Arthur. I didn't know how to address him in my French tache ecrite (creative writing task, wrote Marie's diary entry), so I just called him Meursault and wrote in my justification that we don't know his first name.
I have done it for my WL1 and I do not recall him being called Arthur, either. I would assume that it was just for film purposes. Just stick to calling him Meursault.
Actually, Camus named Meursault "Arthur" in either an earlier version of L'Etranger or in something like a precursor short story about the character. Whichever it was, it was not published until well after Camus's death and I believe it was published as his notebooks and/or unpublished manuscripts.
I just did a little more research and it was in La Mort Heureuse (published in 1971) that Camus had a character called Patrice Mersault (NOT a spelling error) who also kills an Arab but then lives on the dead man's money and eventually dies of a serious illness. So my comment above is incorrect. Sorry.