Efflorescence. Soluble salts that are in Portland cement based setting and grouting materials have migrated to the top of the grout joint. This is not a grout defect, it can happen anywhere, with any cement-based grout, with the most experienced installer to the newest. It more frequently occurs when grouting in cooler weather with high humidity but it can occur at any time. Try a stiff bristled brush dry, scrubbing vigorously, if that does not work, then a Sulfamic acid wash will be required to remove the efflorescence. See Tech Bulletin #000 in the Technical Bulletin section of this web site.
Changes in plane such as vertical corners in a shower must be grouted with a flexible sealant (caulk). Grout cracking is a common problem when the flexible sealant has been omitted in an area that is constantly moving. Use a quality caulking material. Make sure the joints are clean and dry before caulking and allow the caulking to cure for 72 hrs before using the shower. The caulking depth should be at least one-half as deep as the grout joint to adequately provide enough caulking to handle the movement without premature cracking. The caulking requires dry, clean and sound surfaces to bond to. If the tile has a glazed edge, the glaze should be mechanically removed to assist the caulking bond before applying the caulking.
Cement grout typically cracks because of some type of movement in the substrate or tile plane. When new cement grout is soft, it could be from using old grout, high absorption tile, improper curing caused by high or low temperatures, hot-drying wind across installation, failure to damp cure if required and a dry concrete substrate. Pinholes, in cement grout are typically the result of mixing too fast with a mixer and or excessive water in mix and clean up. To correct soft grout, damp cure the grout with plain water by misting the joint for three days, this method has been found to harden soft cement grout. Cracked grout should be cleaned out and re-grouted, taking care to dampen the surrounding grout before placing the new grout. Grout with excessive pinholes and voids will continue to deteriorate over time unless it is re-grouted.
This is typically the result of too much water during clean up, in the grout mix or cleaning too soon (before the grout has "set up" in the joints during installation). This can also be caused by inconsistent absorption of the tile due to glazed edges
If spacers were used to install the tile, were they removed before grouting? This is the most common reason for dark grout joints only at the tile corners.