Problem:
* Incorrect use of "when": "When" is used to introduce a time clause, but the sentence is describing a state (wetness) that affects the moss's slipperiness.
* Missing subject: "It" in "when it wet" doesn't have a clear subject.
Solution:
The sentence needs to be restructured to use the correct verb tense and establish the subject clearly:
* "The green moss was most slippery when it *was* wet."
This version uses the past tense correctly, indicating that the moss was slippery *at the time* it was wet.