As you might have heard, Super Mario 3D All-Stars packs in 3 classic Mario adventures that you can play on your Switch. These titles have been brought over through emulation, and it seems that Super Mario Sunshine shows off one of the side effects of that emulation.
If you take on the "Secret of the Dirty Lake" challenge in Super Mario Sunshine, you'll eventually come to the area you see in the screen above. You might notice tiny grey cubes floating between the red and blue platforms. Those are 'debug cubes' show the path that the platforms will move on. This is actually an error introduced through a bug in the emulation. Those blocks shouldn't be visible.
Nintendo will most likely update the game to remove this error, but I'm guessing most people don't consider it a big deal to begin with.
Thanks to Dondom95 for the heads up!