I'm guessing Dottyback Eggs...they look just like a Dottyback egg mass, and I know you have the three running around in there. Chromis eggs would be smaller and are laid spread over a surface (i.e. a green chromis nest can be several inches in diameter, and the eggs are around 1mm in diameter). I'd stake a big wager those are dottyback eggs.
Also, could be a dropped cardinalfish clutch (i.e. if you have spawning Pajamas).
The section you see was flapping in the current like a thin mint strip or fruit-roll-up, not a mass in a three-dimensional sense. It waved like a flag, and it took a few shots to get what you see. Had I turned off the flow, it would have settled out of sight.
My Chromis usually lay clutch's on flat surfaces, never seen them lay eggs like that.
probably angel eggs, they knew of your plot to remove them and decided to fight the power!