Muscovski is the man who works in the laundromat who owed money to Vlad. When Niko went to collect Vlad's money, Muscovski threw a laundry hamper at him, and tried to escape in a Laundromat, but was chased and pulled over by Niko. He said he would give Vlad his money, but was having trouble getting it together.
It should be noted that like a typical pedestrian in GTA IV, Muscovski's appearance will be different in each of his appearances, not just with his outfit, but also his facial features, lacking or featuring of a mustache, and sporting different eyebrows and hair design. In general, Muscovski can be described as a middle-aged man with a plump appearance. His name means "Moscow's" in Russian. So it's likely that Vlad was simply calling him a "jerk from Moscow" or "Muscovite jerk"
- GTA IV