The Geomancer needs to be on the monster's hate list for Indi-/Geo- debuffs to work. You can't have a party member claim while you have a debuff up and have the debuff apply to the monster. This applies to both Indi- and Geo- spells. You don't have to get on the hate list by acting on the monster though, casting cure on a party member with hate works. Tested with Indi- and Geo- poison.