Tardigrades, known colloquially as water bears or moss piglets, are a phylum of water-dwelling eight-legged segmented micro-animals. He may be scratching his back just like bears do with trees or he may be temporarily stuck to the little bubble because of surface tension, the physics of the microbial world are very different than those of our daily life.
They are one of the most structurally complex organisms of their size. They have brains, nerves, and even simple eyespots. That might not seem like much, but they are so tiny that amoebas are a mouthful to them, which doesn’t leave a lot of room for extra parts.
These little guys are believed to have survived every major extinction event on Earth. There’s more Tardigrade fun over here! (slightly NSFW)