There's this sentence in The Dark Forest by Liu Cixin:
Joel Martinsen translated the sentence like this:
“Look at you, bringing up the issue on your own. Saying you may have killed her, even. We’re just having a casual chat. What’s your hurry? You’re new at this, that much is clear.”
That gives the impression that 嫩主 means something like: greenhorn/novice/newbie/rookie.
Most of the results online for "个嫩主" only bring up the same quote from The Dark Forest, but there are a few others:
It doesn't seem that greenhorn/novice/newbie/rookie can necessarily be directly substituted for 嫩主 here, though.