The moss itself can be removed without any tools. Only problem is that it is poisonous. Henry can't touch it barehanded.
This is the low-spoiler guide for "Removing the moss from the rock" puzzle in Mutropolis. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.