Can't really tell from the picture. Is it that there wasn't complete coverage under the tile with the thinset, and when grouted, it got pushed underneath it? If so, not much you can do except replace. It is critical with glass tile to back butter them with thinset to ensure you have a complete cover, then embed them into the thinset on the wall or floor. Otherwise, you'd usually see the ridges from the trowel, or have a void that the grout could get into. You may get more comments if you post on www.johnbridge.com - that site is devoted to tiling, rather than a general remodeling site like this.