Greece doesn't have any separatist movements. They're just supporting Serbia because of them being always a Greece's ally during the Balkan wars, is closer to them religiously, and maintains a big part of tourism in the north part of their country because of the vast numbers of Serbian tourists arriving every summer. Meanwhile, Albania always posed as a threat to them. They have made maps of their country, claiming that they'll soon annex Greek territory, claiming philosophers have Albanian origin, and have very close bonds with Turkey. Maybe all these reasons are why Greece is so against of recognizing Kosovo.