How do I patch my wall with tile along the seam?
As long as you have secure anchoring of the 6" strip of drywall to the studs behind, and since the strip is along the ceiling, strength is not a major concern here.  Additionally, the primary reason for using joint compound is not for strength but to create a smooth work surface for finishing.

In applications where the drywall will be completely covered, such as with a hard paneling, installation can be finished with no mud/joint compound at all.

In your case, since it's in the bathroom, along the ceiling, as long as the strip is out of the splash zone of the shower I wouldn't bother with mud.

Now if your shower is unusual and the ceiling is in the splash zone, then waterproofing will be necessary and you should use cement board not drywall - but it sounds like drywall or plaster was already used, so replacing it with more drywall isn't a problem in this case.  (Or, if it is, you have much bigger issues already with the bathroom that are outside of the scope of this specific question.)