My first concern is the tile was installed with a gray thinset. A majority, if not all thinsets specific for glass tile are white and include a white powder and a latex liquid additive designed to grab the non-porous glass surfaces. Check your manufacture's specs to confirm the product was adequate for tiles in a submerged environment. If the wrong thin-set was used, water penetration behind the tile will happen sooner than later.
In addition, I would avoid all mesh-backed mosaic tiles and select face-mounted tiles. The mesh backing is made of materials that are susceptible to break down over time especially if (a) your installer failed to properly flatten the trowel notches and back butter the tiles before setting them. Per TCNA P601MB-20, P601 TB-20 and P602-20, you need at least 95% glass surface contact between the glass tile and thin-set. Mesh-backed tiles only reach 75% surface contact and dot-backed 91% according to ONIX. The meshing and adhesive on mesh-backed tiles are the biggest hindrance to adhesion. Neither reach the minimum surface contact required per TCNA standards. Expansion joints per Ej171 are a must under the coping and every 8 to 12 feet along the waterline since glass is the most expensive material in a pool and highly susceptible to thermal shock and cracking quicker more than ceramic/porcelain tile.
As I research the pros and cons of using glass tile and after speaking with glass forensics experts who are seeing more glass tile failures with each year that passes, the more I'm convinced glass tiles along waterlines, water features, and raised spas are high risk for the cost. Submerged glass tiles have a much better chance of success since they are in a more controlled temperature environment. To make matters worse, there are very few pool builders and trades who know how to correctly install glass tile. Many will claim otherwise and install your glass tile using the same methods and materials used to install ceramic, stone, and porcelain tile. My favorite is installers who do interior backsplashes, showers, and floor tiling who think they can start installing glass tile in a pool. A big mistake if they and their pool contractor never read up on the latest TCNA standards. I think this is at the core of the problem in the industry. I would also point to pool builders who wrongly chose volume over quality. As of this post, the pool industry is experiencing the highest number of pool orders in its history because of COVID. Staycationing is at a "forced" all-time high and pools are the solution for many! Unfortunately, the volume of orders well exceeds most pool builder resources to find, vet, train and manage skilled labor to correctly install pools.
According to Dr. Thomas Read of "Read Consulting, LLC" in California, a vast majority of glass tiles failures are the result of thermal shock. Even if you use a diamond blade for glass tile, micro-scratches form at the cut and are often the crack point of origin. Now place your micro-scratched tiles onto a material layer of a tile system (substrate, rendering, grout, adhesive mortar, and glass tile) that all experience different volumetric changes when they dry, or when exposed to external factors like temperature changes and/or humidity. You have multiple points of failure because the free deformation of each layer is limited by the bond between the materials.
If you must install glass... avoid glass tile manufacturers who claim their tiles are pool rated but never provide ANSI A137.2 and TCNA documentation to back that up. Never use recycled glass or glass that is dark, prints on the back, shows bubbles, or see-through. Sun exposure magnifies the dark tile temperature, bubbles or air pockets are easy points where cracks form and clear tiles act like a magnifying glass that heats up the tile and the material behind it.
Lastly, every glass tile I have seen and touched that was installed more than 5 years ago has been etched (roughened surface) most likely from chemical attacks especially acid treatments. It's incredibly important for your pool chemistry to be spot on but we all know that is not a guarantee for reasons that are at times beyond our control. When you look at most pool glass tile manufacture warranties, it tells the story. They are not very strong and they put the blame on everyone else if something goes wrong. Do your homework. Consider your pool's layout when deciding porcelain versus glass. Ask your general contractor how many glass pool projects the tile crew working on your project has done in the last 5 and 10 years. Visit those pools because unfortunately, there are contractors who will lie to get your business. Review all methods and materials used on your project to ensure they are appropriate for the scope of your project. Good luck!