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.
Correct. Don't let the puck just sit there. You would want to move it around slowly over the course of a few minutes to see if it lifts the stain at all. If it does, you know you're on the right track. If the stain gets worse with a puck, it's something else, so let us know.