Ptolemy was indeed Greek (well, actually Macedonian in fact) - the Egyptian Ptolemaic line was founded by Ptolemy i Soter, one of the companions of Alexander the Great. During his lifetime Alexander conquered Egypt, founding Alexandria and the great Library there. When Alexander died suddenly, his wife was pregnant but Alexander had no actual heirs and hadn't named a successor; a fight broke out amongst the companions as they tried to divide up his empire between them. Ptolemy stole Alexander's body and took it to Egypt which he held as Pharoah; Alexander was buried in a tomb beneath the Great Library (although it's not known what happened to the body or the tomb when the Library was destroyed). Ptolemy was the first of the Ptolemaic pharoahs, and Cleopatra was one of his descendants. The Greek influence in Egyptian art & culture of the time is very clear and distinct.
So yes, I'd definitely heard of the name long before Mumsnet. Did you not do Ancient Greece & Egypt at school? I certainly did - at all the schools I attended it would usually crop up somewhere.