Yeah, no
How weird would it be to place a cup of water on a nightstand and find that it has a weird blob floating in it the next day? Things don’t usually just grow that quickly, even inside of a cup of water, so what in the world is going on here?
Here’s one of those rare instances where internet detectives couldn’t offer definitive answers, but their speculations seemed to make a lot of sense. One user said that heat from the OP’s bedside lamp might have melted a piece of plastic, which coincidentally landed inside of the glass. Seems probable, so we’re going with that.