They're on the hard dungeon floor, not the advanced dungeon. You unlock the hard floor by advancing the events with Naevy.
In the hard dungeon, you encounter a demoness on the last map. In the top right of that map are the tentacle plants.
However, even though I am clicking on them nothing is happening tho. How can I get Tentacle Spore from them?
Either you don't have the quest for gathering the spores yet, or you only interacted with them, as you only need to interact with them once to get the spores. If you already have the spores, go back to the guard who gave you the quest.
