I would suggest you lower your water until it isn't above any missing tiles. As imfignewton said once you get one off you can get water behind the others near it and that will loosen those tiles and it will be a cascading effect. Typically you see this when a pool is really full and water gets behind the top tiles loosening them and eventually popping them out.

It won't however hurt the concrete in any way, just loosen more of the tiles near the exposed area. If you can't or aren't going to replace the tiles use a silicone to seal the edges of the tiles that are exposed to stop the water from getting behind the tiles.