Al, now you know why green board is such a poor shower substrate for tile. This will always happen within a few years, no matter how well caulked it might be. The reason is that some water will always get through the grout, and the green board is water resistant, not water proof. You can do repair as already described, but it is a short term solution at best. The long term solution is to tear out the green board and replace it with properly waterproofed installation of cement board, Dens-shield, or even a mortar bed. Not the answer you wanted, I'm sure; sorry.