I've no direct experience with a FPX, but my North Star which I installed myself last year, came with micore board cut to the shape of the face of the fireplace. and that bridges all the gaps to combustibles to the sides and above and below. I don't think You want to cement anything direct to the unit.
So, I just took a quick look at the FPX instal manual, it says you have to supply the board, so you need to have some facing material.
From page 22 of your
units install manual:
"Facing Requirements
NOTE: Artisan faces vary in size. If the facing is over 1” thick (brick, river-rock, etc.) use the
artisan face to create a template.
• The fireplace is shipped with a set-up face that is 1/8" taller and wider than the finished
faceplate. This creates a 1/16” gap around the perimeter of the face when the finished face is
installed. Leave the set-up face in place to act as a template when installing the facing.
• The fireplace requires 1/2" thick concrete-board or other non-combustible to extend from the
header to the base of the fireplace and to the framing members on both sides (do not use
sheetrock or drywall). See Figure 27.
• The non-combustible facing must extend a minimum of 12" above and 2-1/2" to each side of
the faceplate.
• The non-combustible facing must be a minimum 1" thick
• Attach the cement board to the front of the fireplace with screws. Do not penetrate the
fireplace more than 3/4”."
Good luck finding it, Micore is a tough retail find. You usually have to buy sheets from a fireplace shop for $$$$. There used to be a couple online places, still $$ but less than a fireplace shop would sell it for. Ask your dealer or your installers, if they bought a whole pallet that's where it becomes really cheap ad they may have pieces to sell you that you can use.
I would have actually thought, if you had it installed, they would have done that, if you did have it installed, I would ask that it be completed per the manual free of charge. Those things aren't cheap, and if you had it installed, I'm sure that wasn't cheap either, it should have been completed to a state ready for a mason to finish it off.
If you installed it yourself, the manual doesn't say what R/K value the material must have, so you might get away with durarock or something like that, available from HD or Lowes. But again, I don't have FPX experience, so I don't know what it's supposed to be, you might ask your dealer to tell you what material is required.