Glow Lichen is a unique plant type in Minecraft, and to get it, you must search both in the ocean and in various caves in the game. It can also be often found growing on various blocks in the game, such as stone, granite, deep slate, and even tuff.
As well, if glow lichen is left to its own devices, it will continue to grow and spread to other blocks and functions similarly to vines. Players can also further spread the growth of glow lichen if they use bone meal on the plant. Doing so will cause it to spread to a block that is next to it.
That being said, if you want glow lichen, you will need to make a pair of shears enchanted with silk touch, as any other way will just destroy the plant.
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You can make shears using the pattern with two iron ingots placed in the bottom left slot of a crafting table with the other ingot placed in the slot above the one next to the left slot - if you need a visual aide to help with crafting this tool check the image below.
After you get the shears, you will then need to get the silk touch enchantment. To get the enchantment, we recommend trading with merchants such as the librarian until you find the book for it. Once you get the book you will be able to enchant the shears, and then gather as much glow lichen you want from caves.
For more on Minecraft, check out How Long Does it Take for Copper to Oxidize in Minecraft? and How to Get Calcite in Minecraft