I am Manchu and have been to Japan. My feeling is that most Japanese people don't know how many ethnic groups there are in China. They have the impression that Chinese mainland people are regarded as "China people".
Only a few Japanese intellectuals can accurately distinguish the nationalities in China. A Japanese university lecturer I know, Mr. Matsushima, understands these differences among Han, Manchu, Korean and Hui. The Japanese call Manchu "まんしゅじん". According to the Japanese scholars I contacted, they have a good impression of Manchu people in China, especially interested in Manchu folk culture. There are also professional research institutions of Northeast Asian national culture in Japan, and there are frequent academic exchanges with universities in China (such as Peking University and Central People's University) every year.
Of course, these are only in the intellectual class. For ordinary Japanese, they are not interested in China. What impressed them most was the cheap goods, restaurants and the names of some tourist attractions in China, but others were basically unknown.