A musty or "stagnant water" smell caused by decaying food particles or bacterial biofilm in the drain.
Quick Drain Hole Unclogging Guide – for Homeowners blocked fridge drain hole
"Gently poke with a pipe cleaner — never a metal skewer. Flush again. Problem solved." A musty or "stagnant water" smell caused by
Ice forming around the drain hole or on the evaporator coils due to a disrupted defrost cycle. How to Clear a Blocked Fridge Drain Hole - Qeeper blocked fridge drain hole
Note: If you have an older model, the hole might be hidden under a plastic panel. If so, you will need to unscrew the panel to access it.