"What is wrong?"
Translation:Was ist falsch?
I (nativ) wouldn't use "falsch" as 'not-quite-okay' generally. It sounds too strong as 'totally false'. If you want to ask what's factually is incorrect, you'd better use these ones: "Was (genau) ist falsch?" (What's exactly wrong?) or even "Wo liegt der Fehler?" (Where is [lit.: 'lies'] the fault?)
The (short) forms "Was?" / "Was ist?" / "Was ist los?" / "Was ist denn?"... are a kind of slang. They are mainly used if you approach/disturb someone without saying a word. Then he/she might ask you (maybe being nerved) this, meaning "What do you want (from me)?"
On the other hand "Was ist falsch?" / "Stimmt was nicht?" is only used if there is something wrong or you think that or someone gives you a questioning look. However the diffrences are only little...