I think that, behind all that bluster, Moe was scared because he knew the bill for the Corleones' big chunk of money that they gave him to finish his hotel was coming due. He probably slapped Fredo around because he could--he was given to "murderous rages" [novel] and it probably pleased him that he could humiliate the son of the great Vito Corleone in public. That, too, was a bill coming due, and he tried to rationalize it by claiming he had to "kick asses to make it run right."

There is a plaque of Siegel at the site of the old Flamingo Hotel:
https://www.youtube.com/watch?v=N2zOhastXNA