A lot of Greek patriots feel ashamed because of the way the streets in Thessaloniki look, so they ask tourists to visit the city later, after all the graffiti has been cleaned. That’s why they write on the walls, asking tourists to stay home.
In fact, since tourists bring value to the economy and could help municipalities clean up the streets, I would recommend everyone visit this city more often.
And if you are looking for a place to stay, definitely choose Airbnb. Greeks are so welcoming that they even move to cheaper neighborhoods to turn apartments in the city center into Airbnbs.