Now this isn't spoilers or anything it is just my speculation based on typical tropes and the story development so far. Since we haven't seen anyone with a similar mark other then Aldis we can go on the assumption that the mark is something found only on people from other worlds. Furthermore, taking into account Aldis's shrewd demeanor, obvious combat experience, and comfortability with conversing with the opposite sex (Nale and others), and since he mentions in this chapter that he is getting more tired in this world because of "changes to his body" it is not a stretch to assume original Aldis, in his previous life, didn't have the the same body as he has now, or perhaps he is now a younger Aldis. IN CONCLUSION!!!!!! Since the mark is only seen on other worlders (so far) and Aldis does not have the same body as his previous life, then the mark on his forehead must be was binds his soul to his current body. This would also explain why he hides it all the time and it is never spoken of!!!!!!
At least that is my opinion, though we have no info on how he was isekai'd, the theory could hold up regardless of the specific details.