It sounds like you mean the flat, glowing moss on cave walls called glow lichen. Here’s how you get it in Bedrock Edition, step by step.
- Find glow lichen: Look in normal underground caves (and sometimes dripstone caves). Glow lichen looks like a flat patch on walls or ceilings and gives off light.
- Make shears: Open a crafting table and put 2 iron ingots in a diagonal (one in the middle-left slot, one in the bottom-right slot). That makes shears.
- Harvest it with shears: Equip the shears and break the glow lichen. If you use shears, it will drop as an item you can pick up. If you break it with your hand or the wrong tool, it will disappear and not drop.
- Place it where you want: Select the glow lichen in your hotbar, point at a wall or ceiling, and place it. It will stick flat to the block and give a little light.
Quick tips:
- Glow lichen is the "flat vertical" moss you see on walls. If you meant the chunky green "moss block" that's usually only in lush caves, that's different.
- Glow lichen is great for lighting caves without torches and for decorating walls.
If you want, tell me a picture or where you saw it and I can help you find it in your world!