How did this guy get slimelung?

Either you mined something with a very small amount of slimelung and didn't notice (you are very close in the bottom right and top left), or the world gen overlapped with the slimelung biome and left some residual germs (bottom pocket of plants). I would call the latter a bug, but a long standing bug with world gen that effects many things (POIs breaking abyssalite, etc).

 

Edit: The latter can be tested easily if you get the world seed on the ESC menu and start a new save with that seed and check for germs. Technically, even the former because you can screenshot and compare and see if you mined any germ-infested tiles. However, you could only extrapolate which tiles had slimelung spread to them.

Edited May 14, 2019 by tommytom2k2