PCV valve. Looks like all you really need is a new grommet, and poss a VC gasket when you put that on.
Try to be careful when removing the old lower half of the grommet. Use a small screwdrived and some needle nose pliers. If it starts falling apart then it would be wise to remove the valve cover as you don't want the small pieces roaming around in there. They really should not do much damage as they are softer then any other parts under the cover but they could hamper return oil or cause some un-wanted rubbing and such.
You may get a mist of oil on the inside of the hood, but it shouldn't be too bad (assuming you don't have shot rings). I drove for over half a year and cross country with my valve cover port open. No problems, except the fumes getting into the cabin. Not much of a mess at all. Of course, if the screen in the valve cover is really clogged up, that could lead to some spraying.
I really suck at smog.