If it's touching in only one place, something is out of true.
If it is touching the gearcase in one place then the case is out of true. If the contact point is on the prop and as it rotates it wipes the inside edge of the casing then the prop or thrust washer is out of true. However, whilst skimming a wee bit off will be fine, the prop will be out of balance because it is running off centre in the first place.
Could it have taken a knock and the rear edge be distorted. If so, you may get it back into shape by careful use of a hammer.
Generally, you do not want to make the space between the prop and the case bigger than necessary because it will let too much exhaust out into the prop blades.