That is a dust cover. Behind that is a ball bearing assembly, which is probably not designed to be greased or serviced, only replaced.

The dust cover is usually rubber, and you can pick it off using a toothpick or small plastic prybar. Resist the urge to use anything metal on it unless absolutely necessary since this might damage whatever is left of the rubber.

Behind that you’ll see one of two things:

A sealed bearing, which can’t be greased or serviced…

…Or the cage of an open bearing which can be packed with grease.

When I say packed, I do mean packed. That’s how open bearings work. Fill all of the available air space with grease, and if possible push out any of the old dirty grease in the process. You’ll probably want a rubber glove. For particularly annoying to access recessed bearings I use a drugstore oral syringe, i.e. with a wide nozzle and no needle, via filling it using a popsicle stick as a spoon. I use the red Mobil 1 synthetic grease for this type of thing. And, indeed, practically everything else as well such as motorcycle hub bearings, swingarm linkages, etc.

If it’s a sealed bearing that’s trashed you’ll have to replace it which is done via knocking it out from the other side and tapping the new one back in with a hammer, preferably after sticking it in your freezer for an hour or so to shrink it first.