Water slide decal paper, like the kind that comes with model car and airplane kits. After a quick soak in water, the blue backing paper releases from the printed image+carrier+adhesive. We would need to see the blue paper and the cake pan of water for a 100% positive ID.
It’s a starboard decal/sticker. When applying decals it is common practice to spray the surface and sticker with water to allow for repositioning. You can use a squeegie to work the air bubbles out and remove the water to leave the sticker in the desired place. This is very common when applying protection to cell phones.