One of the things I dislike about Thessaloniki is the excessive amount of graffiti. Much of it is meant to be political or humorous, but some of it is very disgraceful.
A lot of non-Greeks who visited told me they thought it was kind of scary to see so much unsightly graffiti everywhere. Don’t be intimidated by it. It’s disgusting but it doesn’t mean your safety will ever be in jeopardy. Take the normal precautions you would take in any city.
As in other major cities, you might even find some impressive murals, although there are much less of them in Thessaloniki than in other cities I’ve visited.