According to a paper from 2010 the official position of China was that it supported peaceful reunification of Korea. But from declarations to practice... And yes, China demands the departure of the USFK. That paper is written based on the declarations of Hu Jintao administration. I'm not sure if anything has changed in the Xi Jinping era in these regards, but I suspect nothing significant.
Unofficially Chinese analysts (quoted in that paper) raised a bunch of concerns, including South Korean FDI displacing Chinese one in the North, and in the long run a unified Korea being a much more serious economic competitor; essentially they fear a combo of cheap labor from North and tech from the South. Another point of anxiety is that a unified Korea might make irredentist claim over portions of the neighboring Chinese provinces that have a significant Korean minority. (China prohibits dual citizenship of the chaoxianzu for the same reason.)