My first thought is that this looks kike a leak from a loose roll, so I'd go with that as a working idea. The cost to find out is the price of another roll, (and you would take out the test roll in total darkness) and the potential to waste that roll.
If you really like this camera, and push comes to shove, then buying a new one would, depending on location, cost $30~$50 and of course, since it is a Holga you could have problems with the new one also.
A PS; I notice that currently B&H has the standard Holga 120N for $33.01 with free shipping. Even if you sort the problem with your current camera, that is fairly cheap for a second camera.