I might change his ability to create additional bodies into one where the daemon can deposit seed pods in fertile soil in which his (obviously disgusting tentacle-branch wielding) offspring can grow to wreak havoc, invested with some of the daemon's warp-power. He uses the offspring to terrorize the surrounding settlements into obedience and to spread his influence in the form of crazy plant growth or a disgusting mold or whatever you want as a recurring theme of his corruption.
This offspring is not as subtle as he is and this is what attracts the attention of the authorities/acolytes. When they investigate, they are confronted by the 'seedling' and must fight it. When they kill it, they can see a blast of green energy leaving its corpse to a point further away (psykers can see the energy beam, psyniscience helps pinpoint the location). The Eldar stone still functions partly and absorbs the corrupted energy of the plant daemon that leaves the corpse of his offspring.
The Acolytes mission (should they accept it) is to hunt and kill all the seedlings, returning the energy to the stone and repowering it. They will need to find out what the stone does: Forbidden Lore Xenos would be handy or they can find some old writings in an abandoned Mechanicum R&D facility. This would also be the ideal place to find anti-plant grenades (I remember those from 40k where they could destroy plantbased scenery).
When enough/all seedlings are killed, the stone will reactivate and the daemon needs to be driven back to it. Will the Acolytes kill its child-worshippers who come to his aid? Will they use the grenades or rather push it back with concentrated fire power? Maybe they can rig and drive a large agri vehicle at it to move it to the stone.