Drywall – What type of drywall do I use around a zero clearance fireplace

As with many textures, it is likely formed from water-soluble base like drywall topcoat. If so, spray it with water from a spray bottle until you think it's saturated enough then take a wide drywall knife and see if you ca scrape it off. If this works, you'll need to tape and re-texture afterwards.

Alternatively, a drywaller can skimcoat over the top of it, then re-texture to your liking. They may often use a giant heavy-nap roller of sorts and "paint" on drywall mud. Then they scrape it flat enough to base whatever texture you intend.

If you could manually scrape off the large peaks before this, though, it will work better.

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.)