What is this green puffing smoke?
Solution 1:
You'll notice that you have a little tiny icon next to your sharpness level. This is the area for buffs and debuffs. This particular one is a debuff, and it lowers your defense, as indicated by the black shield with a down arrow on it. Most likely one of those tiny insects gave it to you; they like to do that.
To clear it out, a Nulberry should do the trick. Or, you can wait until it goes away on it's own.