Based on other occurrences like this one or this one, this appears to be a very strange bug.
This is also documented by a few users in this thread:
Anyone else got this? Was catching a weak Natu and then this things pops up.
Apparently there have been a few weird Ditto-related glitches recently, this being one of them.
The Ditto transformation animation has a separate room with a model ready to load in. For some weird reason, the game loads in these assets when you catch a Pokemon. I've seen a few other screenshots with Ditto in weird positions as well as the room.
For what it's worth, I personally experienced a similar glitch where, after catching a Ditto, every single ball I threw would cause the Ditto reveal sound to play. It did this until I restarted the app.
At this time, the cause is unknown, but the only suggestions I'd have to fix it would be to restart the app and, if that doesn't work, reinstall the app.