Then again, an acro can have no PE and grow like crazy and be happy. Also bare in mind that in the wild many acros don't extend polyps until night time. Check you acros at night. I have some acros that never put out their polyps during the day but at night you can barely see the skeleton through the polyps.
But as a generalization, yes PE = happy SPS. I use PE as a windsock for how happy they are. I sit and move the flow around until I see a colony start to stick out its polyps. Happy. If it gets too much flow, they retract. Not happy.
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