Mystery "Stains" and Liquified Zucchini

I've come to notice that every so often these circular, brownish gray "stains" appear on the hardwood floors in our kitchen and living space. The only thing I can recall spilling is water, and dry cereal so thats not the problem. And they tend to pop up around the same places. The spots in the kitchen wipe right up with a wet cloth, but the ones in the living area require a slight scrub. They look a bit like the residue left after you remove a long term band aid. But they aren't sticky to the touch.

Can you see them? I just scrubbed them away yesterday.

We had hardwood floors growing up, and I don't remember anything like these. Does anyone know what these are? Do you think its mold seeping up through the apartment below us? Gross.

As for the second part of the title, I think I mentioned earlier that we bought some zucchini. I'd been munching on it raw and was planning on cooking some yesterday for dinner. I pulled the leftover zucchini (still in the produce bag) out of the fridge. It still held its shape, but when I reached into pick it up its was completely liquid! I don't know its because I left it in the bag or if it has something to do with our fridge. Our msutard tends to freeze and I have to wipe down the apples and peaches every day because they get soaked in water. But, there are no settings to tamper with on the fridge. Has anyone else had this problem?