What is this?

Basically, I used to store my retainer in this cup with water and one day I went on break so the cup was sitting in the room empty for about a week. When I came back I saw this monstrosity. Considering what is typically stored in the cup, I wouldn't be surprised to find bacteria, but seeing this weird branching pattern caught my interest. I poured water into the cup and it disappeared instantly, so my guess is that it was just some type of bacteria. I know very little about biology so I don't even know if it's possible to identify what bacteria that would be, but at the very least an explanation to the branching pattern would be nice.