Possibly the easiest way to figure it out would be leaving a camera recording the tank for a while after lights out and for a while before the lights come on (or just leave it all night). That said, as I mentioned earlier, I'm leaning towards nudibranch eggs myself, but I don't think those came from a Chromodoris species (and not just because their egg ribbons are known for laying flat rather standing on one side).No other oddballs. Just the normal fighting conch.
For reference, the False Pyjama Nudibranch, Chromodoris annae, egg ribbons are pretty small and look like this: