Keep flushing with hot water until the water runs clear and flows out the back quickly. If the water backs up and sits in the hole, the clog is deeper. Use a small hand pump (or a burst of air from a bike pump) to force the blockage through.
To keep the drain flowing freely in the future: how to clean fridge drain
Remove the bottom drawers (crispers/deli drawers) and the glass shelves covering the floor of the fridge. You don't need to empty the entire fridge, just the bottom section where water is pooling. Keep flushing with hot water until the water
A sour or musty smell caused by stagnant water and mold growth. how to clean fridge drain
Here is the step-by-step process to fix it safely and quickly.