Moss is a material that can be harvested from the trunks of some birch-like Ashenwood trees in many Biomes like Woodlands and Grasslands, but also from Weepwood trees and Mossy Weepwood Logs lying on the ground or in Bog Water in Swamplands without exceptions.
No Power Cell is required to harvest Moss, simply "pull" the parts of the tree that have moss on it or the logs. Usually a piece of Wood (either Ashenwood, Weepwood or moss-less Weepwood Log) plus one unit of Moss will be harvested, rarely only Moss though.
Moss is an useful material to craft Moss Torches at the beginning of the game, and is also an ingredient to craft Mossy Cobblestone, a cubic Building Block that requires finding a rare crafting Recipe (mainly found in Treasure Chests) to be unlocked and crafted.
Moss as a crafting material cannot be placed into the world, cannot be "stuck" to trees nor walls and cannot be put in any quickslot. As Ashenwood trees can be planted from Saplings, Moss is a renewable material. It does not regrow on trees, however it's easy to collect it in large amounts from Weepwood trees in Swamplands.