The fact that President Trump does not like dealing with President Moon should come as no surprise. The other claim from the governor is that Trump said Koreans were “terrible”:

U.S. President Donald Trump told governors in February that he doesn’t like dealing with South Korean President Moon Jae-in and thinks the South Korean people are “terrible,” Maryland Gov. Larry Hogan said Thursday.
Hogan, a Republican, wrote in The Washington Post that Trump had harsh words for South Korea, the birthplace of the governor’s wife, Yumi, during a private dinner sponsored by the Republican Governors Association.
“I don’t remember him mentioning the virus,” Hogan said, “but he talked about how much he respected President Xi Jinping of China; how much he liked playing golf with his buddy ‘Shinzo,’ Prime Minister Abe of Japan; how well he got along with North Korean dictator Kim Jong Un.
“Then, the jarring part,” he continued. “Trump said he really didn’t like dealing with President Moon from South Korea. The South Koreans were ‘terrible people,’ he said, and he didn’t know why the United States had been protecting them all these years. ‘They don’t pay us,’ Trump complained.”
Yonhap
You can read more at the link, but if true based on the context it sounds like Trump wasn’t saying Korean in general are “terrible”, but the Koreans in the government are terrible for not paying more the upkeep of the US-ROK alliance.