Sometime, biofilm and algae can smell terrible. You can use bleach to clean your decoration. Make sure you rinse properly. If your decoration smell bleach after rinsing, rerinse it.
If you cannot make it stop smell bleach, you can use a little tap water with dechlorinator and soak decoration into. Worked for my lava rock that had bad smell and green color.
DON'T put it back in your aquarium if it smell bleach, this can result to nothing to some fish death and/or cycle breaking.
DON'T do anything if this is not a plastic or rock decoration, I don't know what this would do, in this case, wait for another advice