I'm not an expert, but I think a lot depends on where the water hits. If the shower hits this directly (or above it so the water runs down over it) I'd move it up my "to do" list.
If it's to the side, you could probably get away with a temp repair if it's well caulked.
FWIW, the greenboard looks clean and dry, so that's encouraging. Does it feel moist? Are there any soft areas or any other loose tiles?
Edited to add: I answered assuming this was in a shower. Is this shower, tub only, or just wall?
Last edited by Blondesense; 10-18-2012 at 02:06 PM.