Twice now, on two different tanks, I've noticed the following:
In the first few weeks of the tank set up it gets some short green fuzz algae on the back glass. When changing the water for the first time the algae that have been exposed to air dies and the glass is left completely clean.
The algae below the water change line continues to grow but never invades the "purged" area above.
The following water changes don't harm the remaining algae in any way although the water level is being intentionally brought lower.
The tank ends up looking as if you carefully polished the top part of the glass and never bothered to touch the algae below.
That situation continues for several weeks until the plants outcompete the algae and the algae are gone completely.
Yes, I've noticed the same thing with fuzz, dust alga in my aquariums. When I do a water change I also clean the front glass if needed and later I find this alga almost only below the water/air line of water change.
My guess is that this alga or spores reattach on glass very fast or at least faster then I can put the water in.
Algae phenomenon 
reattach on glass very fast or at least faster then I can put the water in.
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