What are the signs of a watermelon past its best?
It sounds like your water melon simply spoiled in the refrigerator.  It is a very perishable food, once the rind is cut off, as it is basically a sugar syrup waiting to be eaten by whatever comes along&mdash;people, dogs, bacteria, yeasts, molds, whatever.

The milky liquid would be waste from the metabolization processes of whatever colonized the water melon.

You are probably wise not to eat it; just discard it.



Some signs of past-prime watermelon include--but all of these border on signs of spoiled, because it is a continuous process:


Aroma:  If you detect an alcoholic, winy, or boozy smell, it is a sign the flesh has started to ferment. 
Another sign is when the rind is not taught and firm.
The melon is light for its size, indicating it may have lost moisture