Hey, Dan -- that sounds like cyanobacteria. It's not an algae, but a bacteria.
Often concentrates in areas with the brightest illumination (hence it appearing
near the surface and at the top of floating plants). It's harmless (as far as I
know), but unsightly. Remove as much as you can. Do some water changes. And then
add Maracyn (an anti-biotic) to knock it out.
Chris Scharpf
Baltimire
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