Considered highly pejorative and insulting to the aforementioned people of these Greek sub-populations, it essentially means "pretending to be Greek" and suggests that their "Greekness" is inauthentic and are thus brainwashed (at best) or traitors (at worst) to the Macedonian, Bulgarian, Romanian or Albanian nations.
The Greek Hellenization process, which was an assimilationist policy of successive Greek Kingdoms and Republics since independence, largely ensured that Greece's minority populations adopted the Hellenic national identity despite being ethnically non-Greek and despite other competing nationalisms in the region. This was extremely significant in a land like the Balkans, where nationalism and belonging are the cornerstone of your whole identity.
2. Everyone knows that the Slavs of Northern Greece are really Bulgarian but one of those grecoman bastards punched me in the mouth when I told him he wasn't really Greek.