Left my 3DS unused for months and it developed weird internal yellow stains

Hi everyone,

I’ve had this Nintendo 3DS for years and it was always white and clean. About six months ago I left home for a long period, and when I came back and picked it up again, I noticed this weird yellowing pattern on the top shell.

The stains seem to be inside the plastic, not something sitting on top. So I tried partially opening the console to see if I could access that layer, but that outer piece didn’t seem removable (or at least not without risking breaking something), so I stopped.

Is this humidity related? Is there any safe way to fix or reduce it, or is replacing the shell the only option?