it could be a cyst like marydean described, there is a special name for them, or it could be a tooth.

DS' teeth always bugled outward not upward when they were coming thru, but we nenver actually saw white at the front/side, just at the top when they cam thru.

Maybe get your MCHN to have a look? You could go to a dentist if you wanted to after that, but some free advice may be just as reassuring.