When you open the block editor and click on a tile, you should see what looks like two tiles squeezed together to make a 32x16 block. Each tile is FireRed is made up of two layers; a bottom layer and an overlapping layer. The one of the left is the bottom layer and the one on the right is the overlapping layer. This is how the game is able to put to completely different kinds of tiles together in one.
That having been said, the bottom layer of a tile is automatically covered by the Hero sprite, so all you need to do to fix your problem is move the grass tile to the left side of that 32x16 and make sure that the right side is blank.
OR you could click on the block, find where it says background byte, click on the drop down box, and click "Block is covered by Hero".
Either should work.
