Left crank grommet pops out of hole
 

Is this grommet solely a dust seal? It should fit in the ring slot around the crank arm and within the bottom bracket.

I tried pushing it back into place, but it just pops/slides loose again--without the cranks spinning. If they spin, it comes right out.

How urgently do I need to pull the crank arm to address this? Rather, what is causing it?

I'm having trouble understanding where this is. Can you post a pic or find a similar one online?

Are you talking about something like this: http://www.sickbikeparts.com/catalog...s/Dust-Cap.jpg
or this: http://t0.gstatic.com/images?q=tbn:Z...C_0205.jpg&t=1
or this: http://harriscyclery.net/merchant/37...arge/CK029.jpg ?

Neither of those first two, from what I can tell. It's these exactly:

http://i1023.photobucket.com/albums/...loosegrom2.jpg

http://i1023.photobucket.com/albums/...loosegrom1.jpg

Pull crank arm, unscrew lock ring , remove cup there will be a groove in the cup for the dust shield to go into. (maybe)
see if you can fit that ring back in there again.

that far in , you might as well do the rest.. overhaul the BB .. clean fresh grease and new balls...

[ no need to remove other cup just clean , grease and drop bearings into grease.]

That's seal-ring for the bottom-bracket. May have been installed backwards.