No confirmed fix yet. Players suggest: It's very early on in the game so if you get through the cave without it spawning you can start a new run and try again.
The problem
I've already finished hidden cave 2 times and on neither occasion have I met the moss troll, just some quirky goblins screaming I gound their secret stash, and empty box a second time (I was so mad I desotroyed it)
What players suggest
You mean the Mushtroll? Like a lot of stuff it will appear randomly. It's very early on in the game so if you get through the cave without it spawning you can start a new run and try again.