yea Jerry, i figure most of us are experts-by-experience when it comes to string algae but none of us really understand it!! my experience is that it shows up everywhere but is a bit less in the pond with its filtration system than in the tubs. i wouldn't expect moving a 'cleaned up' plant from the tub to the pond to cause a sudden explosion of algae, but i wouldn't expect the algae to just disappear either.
let's face it Jerry, sometimes you're just an unexplainably lucky guy!!
(ok, i could go into a long diatribe about karma and how it's not really so unexplainable that you're so lucky, but..
)