Hook: Floor Small (Green Square)The Toxic Phosporescent Stem decoration fits into a small green square floor hook.
Hook: Floor Medium Narrow (Blue Rectangle)The Toxic Phosporescent Stem decoration fits into a narrow blue medium rectangle floor hook.
Hook: Floor Medium (Blue Square)The Toxic Phosporescent Stem decoration fits into a blue medium square floor hook.
How to Get a Toxic Phosporescent Stem Decoration
Every time you complete your personal conquest, you receive a lockbox with three random conquest Crafting Node Decorations, one of which could possibly be a Toxic Phosporescent Stem. You get a second lockbox if you complete your guild conquest goal as well.
If you're looking to get a Toxic Phosporescent Stem and haven't gotten lucky with your conquest rewards, you can buy one from the GTN from another player. These decorations are farily inexpensive because there is an unlimited supply - more are created every time someone finishes their conquest goal.