Is the tile grout white?
If you look up "Pool Patch Tile Grout" online, you will get lots of hits for white and grey tile grout. For small holes like that, be sure the surrounding tile is clean and the hole is free of debris and dry (lower the water line below the tile line), then apply the grout to it. You can even tool the grout line a 1/4" above and below the hole to try to blend it in better.