Your photo period should be around 6 hours daily, a step down from the original 8. alternatively, consider spliting the photo hours to 3 hour cycles with an hour dark period in between as an experiment.
Another view to take is the tank is too sparcely planted and not able to sufficiently take advantage of the light available.
Most floaters can tolerate a bit of surface agitation unless your water is flipping them upside down and all around which it shouldnt. One that's easily disturbed would be salvinia (spangles) that turns brown when submerged for a few days.
What does disappearing mean? are they being eaten or are they suffering from necrosis, or turning yellowing then dying?
Mind you, cutting back on the light period wont completely eliminate the algae. it only serves to stop its spread. If you are apprehensive in using hydrogen peroxide or excel in the tank, you can use the dip method. Take the plant out and dip in a 75/25 solution h2o2/water for 30 seconds, rinse with clean water, then replant in the tank.
Its one of the safer ways to deal with algae without directly affecting your water column.
"I am Groot", the faithful protector