It's not coming off because it's not a "grease spot." It's actually wear from your fingers rubbing the surface repeatedly. If you take a look at most keyboards, they often have a similar wear pattern on the key tops, particularly the space bar right about where your thumb repeatedly hits it. In effect, you've "polished" the trackpad surface.
It won't go away without replacing the whole top cover. However, you can conceal it (and help prevent the wear from getting worse) by applying a TrackPad cover.
When I use my macbook I usually use it on my desktop so I'm using external keyboard and mouse which keeps my trackpad pristine.. One way to keep it clean but not really utilizing the laptop aspect of it.. lol
I don't mind the wear spot. My question is, would the track pad cover help with dust and, er, crumbs? I've never used one. Are the keys as responsive with one on?