How can I fix this crack in tile floor

Remove that one tile and replace. If the floor is soft under that tile and fridge, you want to look into a bigger problem.

How to Repair Holes or Cracks in Ceramic Tile

Patching unwanted holes in glazed tile like this can be a tricky proposition. Here's a solution that may not always be perfect, but will make a big improvement. Polyester resin or auto body filler, often used to repair car bodies. Mix up a small batch by adding some hardener and stirring it thoroughly. Using a putty knife, press the resin into the hole slightly overfilling it. Then just as the material begins to harden while it's somewhat rubbery, trim off the excess with a razor blade.

Next use 600 grit sandpaper to sand the filler smooth. Wetting the paper with water makes it work best. Now it's time to try and match the color of the tile. A porcelain touch up kit which is available at most home improvement centers comes with several different colors of paint. You can also use artists acrylic paints available from Michaels, Hobby Lobby, art supply stores, and online.

The trick here is to mix these different colors together until we come up with one that matches the tile. Apply the paint with a small brush. You can test it on a nearby tile. When you get it the way you want it, then just paint over the patch and let that dry. Give it one final sanding with that very fine sandpaper and you'll have, well, maybe not a perfect match, but something that's a heck of a lot better than what you started with.

Fill the crack in the floor tile with silicone caulk (make sure you get the brand that cleans up with water). The caulk will not separate if the tiles moves a little. Using white caulk works great if you can not get the color of tile. You look at the crack and see the dark void that is the crack. I have done service on kitchen cabs for 22 years. Crown or light valance miter that is open just a little bit, the homeowner sees that dark void. Make to dark void go away and the homeowner is happy.