Last week I bought a watermelon, and that day cut the rind of it. Also that day it was placed in a glass container, with a plastic lid that fits snugly around the glass on top, and stored in the refrigerator.

Today I went to finish it off, after not eating some for a few days, and was greeted by a 1/4 inch of milky white colored watery liquid in the bottom of the container, along with partly mushy watermelon innards.

I don't intend to eat it, but have had slightly mushy watermelon innards before, without the liquid described above. Anyone have any ideas what that is?

Additionally, what are other signs an already-cut watermelon is past its best (but not spoiled)?