As Michael reminded Moe, the Corleones gave him a "big chunk of money to finish your hotel," so Moe knew that Michael's visit meant the bill was coming due. I think he hoped that he could cut a deal with Michael that would leave him in control of the hotel, as he claimed he had talked to Barzini about. I don't know if he actually expected Michael to agree, but he figured that his bluster would would save the day for him. As Oli pointed out, he thought he could cow Michael because he saw the Corleones as weak.

I think the reference to "skimming" didn't mean that the Corleones were in on the skim, or that Moe wasn't giving them their share. The context was that Moe was losing money on the hotel--he brought up skimming, as if Michael believed that the hotel's losses were due to the skim going into Moe's pocket.