Mr. Godfrey Ablewhite "loved everybody. And everybody loved him" (220.127.116.11). This is Gabriel Betteredge's initial description of Godfrey Ablewhite. We know that Betteredge has known Franklin Blake for ages and thinks very highly of him, but, in Betteredge's opinion, Franklin has nothing on Godfrey:
[Godfrey] was, in point of size, the finest man by far of the two. He stood over six feet high; he had a beautiful red and white colour; a smooth round face, shaved as bare as your hand; and a head of lovely long flaxen hair, falling negligently over the poll of his neck. (18.104.22.168)
In other words, Godfrey looks like a stereotypically handsome English gentleman: tall, blond, athletic, with a good beefy complexion. Franklin, on the other hand, is shorter and has a kind of foreign look to him from having lived abroad for so long. And in addition, Godfrey is famous for being a philanthropist, or someone who is very active in charities. He is a lawyer (a "barrister" is an English word for the kind of lawyer who files briefs, but doesn't argue in front of a court), and he is also a "ladies' man by temperament" (22.214.171.124).
Generally, being called a "ladies' man" wouldn't be much of a compliment – it's too much like being called a "player." Of course, Betteredge only describes Godfrey as a "ladies' man" because Godfrey's charities are all for women's causes. But it's interesting that he should call him that, since we find out later that Godfrey has at least one secret mistress, and most women find him irresistible. So Godfrey really is a "ladies' man," and not in the way that Betteredge meant.
At the end of the book, we find out that Godfrey's public face is a total lie: he's keeping up the charade of being a philanthropist in order to keep people from realizing that he's a fraud. His whole public life is a disguise to hide his hypocrisy. So it's appropriate that Godfrey is wearing an actual, literal disguise when Cuff and Franklin Blake discover the truth about him:
He traced with his finger a thin line of livid white, running backward from the dead man's forehead, between the swarthy complexion, and the slightly-disturbed black hair. 'Let's see what is under this,' said the Sergeant, suddenly seizing the black hair, with a firm grip of his hand.
[…] It was—GODFREY ABLEWHITE. (126.96.36.199, 183).
It makes sense for Godfrey to disguise himself when he goes to the bank to redeem the Moonstone – after all, he could probably have guessed that Cuff would be watching the bank to see who picked up the gem from Mr. Luker. But the dramatic "unmasking" of the dead Godfrey Ablewhite works as a symbol for the figurative unmasking that is happening, as well. Franklin Blake and Sergeant Cuff realize that Godfrey Ablewhite isn't as godly and charitable as he wanted everyone to believe.