One of the 0x50 sub-tiles indicated in the blue square.
The 0x50 sub-tile is a sub-tile in Pokémon Red, Green, Blue, and Yellow with an index number of 80 (hex:50). A sub-tile is part of a tile of 4×4 parts, itself part of 4×4 tiles known as a tile block.
The sub-tile varies based on the tileset for the current map. For example, in Route 1 it is the bottom-left corner of a bush, while in Viridian Forest it is an 'up arrow'.
This sub-tile is needed for buffer overflow techniques that have the game attempt to copy saved screen data into another buffer until the first 0x50, such as CoolTrainer♀ move corruption and LOL glitch.
Without the player having a 0x50 sub-tile on the screen, it would likely cause the game to freeze during the corruption due to the game copying too much data.
List of 0x50 tiles