Yep ... an inconspicuous spot in definitely the first step ...

 

Gotta remember too, denim reacts to pressure and polish. Anything aggressive you might get swirl marks or "polish" the matte finish. The question of decal marks like this in Denim paint are more of "Is it adhesive" or "Is it scratched". Basically for denim, best thing to do is whatever will require the least of amount of manual labor. Goo Gone should work well, if flat and level, apply and just let it sit (try an obscure spot on the paint first, I doubt it'd react with the paint but you never know). If it's a vertical piece, soak some microfiber and let it sit on the spot. Once a few minutes have passed, clean microfiber cloth, wipe it gently, see if it's improved. If it has, just continue until the adhesive is gone, then a wipedown with denim cleaner or Wizards wipe down for Matte paint.

 

Be very careful and don't rub it very much or the denim paint will start shinning.