I couldn't get the new greenboard all the way down behind the tile because the concrete set was obstructing. The tile was apparently placed on after the drywall. Anyway, I have a slight opening(approx 1/8 to 1/4in) from top of the tile to the greenboard, which varies somewhat because of the set. Whatever I use to seal that area is not gonna be that noticeable but I do want it to be sealed and watertight. I thought about applying some sanded grout on top of the tile as fill, seal and then caulk with silicon caulk. Does anyone have anything better that might work or is this OK? I'm getting pretty close to wrapping this project up. Thanks, Jim.