Sort of depends on how long the leaf (organic) was allowed to sit in that spot and transfer the stain to the plaster. An increased FC level can remove organic staining, however it may take a while. Liquid chlorine or bleach is fine to use if you don't want to over-work your SWG. As a test, have you tried rubbing a chlorine tablet against the stain to see if it changes? Just to confirm it's organic? But yes, if you increase the FC level, perhaps about halfway to SLAM for a while with consistent brushing, you should see a difference.