Green Tree Frog dead. Why?

[Shelilla]

I'm sad to be putting this as my first post in this forum but I need to know what killed my Dad's Green Tree Frog.
I was looking after it while my parents were on vacation. The past two days the only thing I noticed odd about it was that it was sitting pretty low on the glass/looked like it was having trouble not slipping off. Otherwise, it appeared perfectly healthy and fat.
The last time I saw it alive was I think yesterday morning. It was sitting on the corner of the water container. This morning I found it dead and floating in the water.

The tank it was in has another Green Tree Frog, so I would really like to know now if what killed it could also kill the other one, because I don't want that to happen.

As far as I know, this frog was at least a few years old, so is it possible it died of old age?

And one major thing I noticed taking it out of the water was that it had a noticeably large, dark blue spot on it's side. I never noticed anything like this before it died, so could this be the key to its death?

I know this is morbid, but for visual reference here are some pictures.

Blue Spots:

Stomach:

Back:


Rest in peace, beloved amphibious friend.