Its high temp brake grease, likely silicone grease.

 

Yeah, looks like a sloppy application of brake grease.

 

In case you don't know, this grease is usually applied to the back of the new brake pads to avoid squeal often associated with new brakes. Looks to me like they might have greased the anti-rattle clips as well. Probably not an issue, but not something I'd want done either. Either way this grease won't last very long. If your Jeep stops well I wouldn't worry about it.

 

I'd wipe off the excess so that it doesn't somehow end up on the face of the pads.

 

You're supposed to grease all metal to metal sliding/contact points on the brakes.
Pins/Bushings
Anti-Rattle Clips
Pad Abutment and guides

Of course the piston moves, it has a dust cover, the brake fluid lubes it, you'll create a big problem if you try put grease in there.

Grease between the pad backing/shims and the caliper, may or may not prevent noise. I always use it, but quality pads with the anti-rattle shims, the shims are suppose to do that, still the grease shouldn't hurt, so I apply it.

 

This ^^ Especially on cars that do not have replacement clips and the pads rest directly on the caliper brackets. I've found lubing the back of the pad can allow the pad to rock in the caliper and cause unwanted noise so I avoid that (not experienced with a WK2 but on other vehicles).

On XJs you're either welding and grinding the slip surface or replacing the knuckle if it's not lubricated and maintained over time.

 

On XJs you're either welding and grinding the slip surface or replacing the knuckle if it's not lubricated and maintained over time.

LOL, I fixed up an old XJ years ago for my daughter to drive in H.S. And I had a mig welder filling in the big rut ground into the rails for brake pads. Ground it down, and filed it flat, it worked like a charm.