Bicycle Mechanics - 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?

It should fit in the ring slot around the crank arm and within the bottom bracket.

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/images/Dust-Cap.jpg
or this: http://t0.gstatic.com/images?q=tbn:Z-Ps5AiMNE-2xM:http://www.tokyofixedgear.com/img/pds2/medium/DSC_0205.jpg&t=1
or this: http://harriscyclery.net/merchant/370/images/large/CK029.jpg ?

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

http://i1023.photobucket.com/albums/af354/deadprez012/diamondback_loosegrom2.jpg

http://i1023.photobucket.com/albums/af354/deadprez012/diamondback_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.