Tear in tire shoulder, what can I do?

Good thing you posted a photo. That cut is not significant, but I don't see the bubble you're referring to. This should be inspected at a tire and alignment shop. There is going to be a depression along the sidewall on any tire, but some are harder to find. That is where the outer-most belt ended, and is normal. If you're seeing a "tumor", about the size of an egg, that can indicate air leaking through the inner layers and building up inside the outer layer. That damage is caused by a hard impact, like hitting a curb or large pothole.

There appears to be some distortion in the photo just to the left of that cut. If the tread actually is shaped like that, there could be a shifted belt, and that IS cause for tire replacement. Also, if you look at that rib that travels through the cut, the two ends don't line up. That is another potential clue to a shifted belt. Eventually you will feel that as a wobble. If it's a front tire, the steering wheel will shimmy back and forth, and you'll see and feel that the easiest at slow speeds, like when in a parking lot. If that's on the rear, you'll feel the seat shimmy back and forth.

If it is determined there is no shifted belt, the tire should not be unsafe. The cut is not deep enough to reach the belts or structural layers. It is just in the outer layer that is molded to the carcass.