Stress marks a fairly common among beardies, Bowie had them quite often when I first got him, but they disappeared almost completely after about a month of him settling in. Keep in mind they may not always be an indicator of stress either, I've noticed bowie only has them now when he's basking - perhaps trying to darken his colour to absorb the heat. Some beardies express them when they're happy, some beardies express them when they're hungry and some just express them, the usually go away from anywhere to a few minutes to maybe an hour or so. Out of curiosity, what's your husbandry like? in the mean time, as long as you're keeping an eye on your beardie, making sure his/her diet, stool other habits are normal, it shouldn't be an issue. Usually stress marks are only an issue when they're constantly displayed.