Solution would be to partially fill the gap about a half way with grout and to finish of read seal remaining with commercial grade 100 silicone caulk.
Gap between shower floor and wall tile.
If your gap is smaller than 1 8 inch in width use regular caulk.
Another option would be to fill the gap with grout.
However due to different rate walls and floors expand grout will eventually crack again leaving you with the new hairline crack.
Leaving a 1 16 gap between the tub and tile will prevent the tile from cracking when the tub expands.
Similar to the gap that can exist between your wall tile and tub you may have a space between the floor and your bathtub.
Or fill the gap with grout.
This joint should then be filled with 100 silicone to allow for expansion and contraction as well as providing a waterproof layer.
In addition leave a 1 16 to 1 8 expansion joint between the main wall tile and shower floor tile or adjacent wall tile.
Fill in the gaps between the tiles using a rubber grout float.
Fill gaps larger than 1 8 inch.
Grout the gaps between the wall tiles take out the spacers and prepare the sanded grout mixture.
And after installing the floor tile you can install that final or first row of wall tile.
Place the 1 16 horseshoe shim on tub and measure up to the laser level.
So to accommodate the flooring tile many like to leave a small gap between the last row of wall tile and floor.
Soak a sponge in water and wet the tile with it.
Plus a 1 16 gap allows the caulk to properly fill the space and waterproof the transition between the tile and tub or shower.
Grout is generally the best choice for filling joints between tiles in showers or elsewhere.
Since you are joining two different materials together you should use caulk in the space where the floor tile and tub meet.
If you have tile on the wall adjoining the tub select a caulk that matches the color of the grout.
But where two tiled shower walls meet or where a shower wall meets the floor called change of.
Hopefully these tips help you with your project.