I think “mesi” is the root for “middle” or “center”. If that’s the case there might be several of them scattered around Greece. I seem to remember reading something in Herodotus where he’s bad mouthing Thracians and mentions Mesi.
Herodotus mentions Mysoi, the inhabitants of Mysia, an area in N.W. Asia Minor. Later on there's a group called the Moisoi or Moesi in the Balkans (in modern northern Bulgaria and eastern Serbia), who gave their name to the Roman province of Moesia, but the first mentions of them seem to be later.