Flat tire: white liquid inside the tube
There is a type of whitish liquid sealant that some folks (shops, manufacturers) put inside tubes, so that if you have a small puncture it is autimatically sealed by the liquid, which solidifies around the tiny puncture.

From what you say, it is not possible to say if that is the case, or if the puncture is larger and water went inside the tube. That would theoretically allow for the powder inside the tube to form a gooish liquid, although I doubt there would be enough amount of water for it to be noticeable.

From my experience, patch glue doesn't mix well with these white sealants, in every tube I patched that already had this liquid the patch got loose soon after, resulting in another flat. So I gave up these sealants forever, unless something new and different appears.

I would suggest, in this case of doubt, that you use the tube's rubber for another stuff (tying something at home ;oP ) and buy a new tube. That is not so expensive, and buyng new tubes is integral part of cycling, anyway. Besides, you could cut the old tube to look inside it and get more info which could save you future trouble (Where did this tube come from? Would other new tubes contain the same liquid? Was it sealant or just water? Etc.)

Saving old ones for "reuse" too, they are much more useful in the basement than in the landfill.